Comparative Study of Multilayer Hard Coatings Deposited on WC-Co Hardmetals

Abstract

This paper examines the impact of a multilayered gradient coating, applied via plasma-activated chemical vapor deposition (PACVD), on the structural and mechanical attributes of nanostructured WC-Co cemented carbides. WC-Co samples containing 5 and 15 wt.% Co were synthesized through a hot isostatic pressing (HIP) process using nanoparticle powders and coated with two distinct multilayer coatings: titanium nitride (TiN) and titanium carbonitride (TiCN). Nanosized grain formation without microstructural defects of the substrates, prior to coating, was confirmed by magnetic saturation and coercivity testing, microstructural analysis, and field emission scanning electron microscope (FESEM). Nanoindentation, fracture toughness and hardness testing were conducted for uncoated samples. After coatings deposition, characterizations including microscopy, surface roughness determination, adhesion testing, coating thickness measurement, and microhardness examination were conducted. The impact of deposited coatings on wear resistance of produced hardmetals was analyzed via scratch test and dry sliding wear test. Samples with higher Co content exhibited improved adhesion, facilitating surface cleaning and activation before coating. TiN and TiCN coatings demonstrated similar roughness on substrates of identical composition, suggesting Co content’s minimal influence on layer growth. Results of the mechanical tests showed higher microhardness, higher elastic modulus, better adhesion, and overall superior tribological properties of the TiCN coating.

1. Introduction

Tungsten carbide–cobalt (WC-Co) cemented carbides known as hardmetals, have earned widespread recognition for their exceptional hardness, thermal stability, and wear resistance, rendering them indispensable in various industrial sectors, from cutting tools to wear-resistant coatings [1,2,3]. Despite their impressive attributes, WC-Co hardmetals are susceptible to surface damage and increased wear, especially under harsh operating conditions [4]. Consequently, there is a compelling need to develop advanced coatings capable of enhancing their performance and durability.

Recently, significant attention has been focused on the plasma-activated chemical vapor deposition (PACVD) method for applying functional coatings onto WC-Co substrates [5,6,7]. This technology offers a unique blend of the advantages of both chemical vapor deposition (CVD) and physical vapor deposition (PVD). Operating at lower temperatures, PACVD prevents the formation of the unwanted eta-phase, a common occurrence in CVD processes above 550 °C, and eliminates the need for sample rotation, which is characteristic of PVD. PACVD techniques provide a versatile approach for depositing thin, hard coatings onto hardmetals that offer improvements to wear resistance, hardness, and thermal stability [8,9,10].

The utilization of PACVD treatment for coating nanostructured hardmetals is preferred to other coating techniques due to its ability to operate at lower process temperatures [11,12]. Nanostructured hardmetals typically have finer grain sizes and complex microstructures, which can be adversely affected by high-temperature coating processes. By employing PACVD at lower temperatures, the risk of grain growth and microstructural changes in the substrate is minimized, ensuring that the desirable mechanical properties of nanostructured hardmetals are maintained. Additionally, lower process temperatures reduce the thermal stress on the substrate during coating, leading to improved adhesion between the coating and the substrate, ultimately resulting in enhanced performance and durability of the coated components [12,13].

Within this domain, titanium carbonitride (TiCN) and titanium nitride (TiN) multilayer coatings have emerged as highly promising options due to their superior mechanical properties, strong adhesion to substrates, and enhanced resistance to wear [13,14,15,16]. Multilayer coatings, featuring alternating layers of various materials, have been attracting interest for their capacity to customize properties like hardness, adhesion, and toughness. This enables them to achieve improved resistance to wear and mechanical stress. Current research on coating hardmetals focuses on achieving a delicate equilibrium between hardness, toughness, wear resistance, and other sought-after properties tailored to fulfil the precise demands of industrial applications [17,18,19,20]. By advancing coating technologies and understanding the underlying mechanisms governing coating–substrate interactions, significant performance improvements in WC-Co hardmetals are anticipated.

As many studies have confirmed, the major factor influencing the behavior of the substrate/coating system is the interface between the coating material and the substrate surface. This bonding interface is influenced by the chemical compatibility between the coating and substrate and the potential formation of intermetallic compounds at the interface. Additionally, the mechanical properties of the coating, such as its hardness and elastic modulus, are crucial in determining its adhesion to the substrate. A mismatch in mechanical properties between the coating and substrate can result in issues such as delamination or cracking, thereby reducing adhesion [16,17,18,19,20]. The surface roughness and cleanliness of the substrate surface also play significant roles in coating adhesion. A rougher surface offers more contact points for mechanical interlocking between the coating and substrate, thereby enhancing adhesion. Contaminants or oxides on the substrate surface can act as barriers for adequate coating adhesion. Postdeposition treatments, including annealing, surface cleaning, and pretreatments, play a crucial role in influencing coating adhesion. Annealing treatments can reduce residual stresses within the coating–substrate system, thereby decreasing the risk of delamination or cracking and ultimately enhancing adhesion. Pretreatments, such as substrate roughening, cleaning, or interlayer deposition, enhance mechanical interlocking between the coating and the substrate, leading to improved adhesion [12,13,14,15]. Overall, coating adhesion to WC-Co hardmetals depends on various factors related to both coating and substrate properties.

Nanostructured hardmetals used as substrates in this study exhibit several distinctive features that set them apart from conventional counterparts. The finer grain size enhances mechanical properties such as increased hardness and strength while maintaining high toughness due to the presence of nanoscale grain boundaries. These nanostructured carbides provide exceptional wear resistance, making them ideal for cutting tools, wear-resistant coatings, and applications where resistance to abrasive and adhesive wear is needed. One of the main advantages of nanostructured cemented carbides is the ability to tailor their properties to specific applications through precise control of composition, grain size, and microstructure, enabling the development of customized materials optimized for specific exploitation requirements [21,22].

The most recent research on hardmetals with coatings covers a range of aspects, such as novel nanotechnology approaches, refining coating deposition procedures, employing advanced characterization methods, and innovating new coating materials. The final goal is to enhance performance and broaden applications [23,24,25,26]. In this research, nanostructured WC-Co cemented carbide samples containing 5 and 15 wt.% Co were fabricated using the sinter-HIP process. Employing an advanced surface engineering technique, this research focuses on enhancing the mechanical and tribological behavior of the carbide substrate. This investigation involves applying complex TiN and TiCN coatings using the PACVD process. This paper is part of comprehensive research in developing new, innovative coatings on nanostructured hardmetals. The development of new types of multigradient coatings on tungsten carbides with improved hardness aims to enhance the performance and durability of various industrial tools, from cutting tools to mining and construction tools. By developing coatings that further increase the hardness of tungsten carbide, it is possible to produce tools that offer improved performance, longer service life and reduced maintenance requirements, which would ultimately result in cost savings, increased productivity, and enhanced reliability of tungsten carbide-based materials.

2. Materials and Methods

2.1. Sample Production Process

The samples of nanostructured hardmetals were obtained through the sinter-HIP process, a method extensively described in the literature [27]. This process, conducted under vacuum conditions, involved single-cycle hot isostatic pressing. In this technique, mixtures comprising nanosized tungsten carbide (WC) powder (grain size: 0.095 µm; specific surface: 3.92 m²/g), sourced from H.C. Starck in Goslar, Germany, and cobalt (Co) powder (grain size: 0.640 µm; specific surface: 2.96 m²/g), from Umicore in Markham, ON, Canada, were utilized. Additionally, vanadium carbide (VC) from Umicore in Markham, ON, Canada, and chromium carbide (Cr₂C₃) from H.C. Starck in Goslar, Germany, were added as grain growth inhibitors (GGIs) to the mixtures. Two distinct mixture compositions were generated, varying in cobalt (Co) content, with 5 and 15 wt.%, respectively. For each powder mixture, the content of grain growth inhibitors and the carbon ratio were individually adjusted. By adjusting the carbon content in the sintering atmosphere, the goal was to avoid the occurrence of free carbon and undesirable eta (η)-phase constituents. The carbon ratios for the mentioned mixtures were 0.275% C for the WC-5Co (mixture with 5 wt.% Co) and 0.150% C for the WC-15Co (mixture with 15 wt.% Co). The sintering process was implemented at a temperature of 1350 °C under an inert argon gas pressure of 10 mbar for 30 min in an FCT Anlagenbau GmbH furnace (Sonneberg, Germany), type FPW 280/600–3-2200-100-PS. Subsequently, warm isostatic pressing was conducted for 45 min at an Ar pressure of 100 bar.

The PACVD coating of hardmetal substrates was carried out using a system manufactured by Rübig GmbH (Marchtrenk, Austria), type PC 70/90. Prior to coating, the sample surfaces underwent treatment to enhance the adhesion of the applied layer and subsequent characterization procedures such as microhardness, thickness, and roughness testing, as well as Rockwell adhesion and scratch testing. The samples were subjected to ultrasonic treatment by immersion in a bath of 99.8% isopropyl alcohol for a duration of 5 min, followed by drying. Before coating, a 2.5 h ion-dusting treatment was conducted on samples at a temperature range of 490 to 530 °C. During the ionic cleaning process, the voltage difference between the anode and cathode was carefully regulated, ensuring ion particles struck the surface at high velocity, effectively cleansing the sample and initiating surface activation processes. This standard cleaning procedure involved a gas mixture consisting of 13% N₂, 4% Ar, and 83% H₂, applied at a voltage of 540 V and a plasma power ranging from 900 to 1800 W, all under a pressure of 2 mbar.

During the TiN coating process, precise control over various process parameters was maintained, including temperature, gas flow, voltage, plasma power, pulsation time, and pressure. High-quality pure gases (H₂, Ar, N₂) and the precursor TiCl₄ were utilized in the coating procedure. The TiN coating was applied for a duration of 6 h at 530 °C. Following the coating process, the samples were gradually cooled to room temperature using a hydrogen flow rate of 100 L/h for 1.5 h [28].

For the TiCN coating process, strict control over process parameters similar to those of the TiN deposition process was upheld. High-purity CH₄ (quality 5.0) was introduced alongside H₂, Ar, N₂, and TiCl₄ precursor gases. A complex TiCN coating structure was deposited, comprising a thin supporting layer of TiN succeeded by a layer of TiCN. These layers were alternately arranged, with TiCN serving as the final layer. The TiN coating serves as a supportive layer when the TiCN coating is applied, aiming to reduce residual compressive stresses at the interface between the coating and the substrate and enhance coating adhesion. Available studies have demonstrated that the TiN layer significantly reduces residual stresses in multilayer coatings due to a smaller difference in the thermal expansion coefficient between the TiN coating and the base material [29].

Twenty layers of TiN/TiCN were meticulously applied to enhance coating hardness, alleviate residual stresses, and strengthen interlayer adhesion.

The deposition temperature throughout the coating process was maintained at 530 °C. The transition from the TiN to TiCN layer was achieved through modulation of N₂ content and the introduction of CH₄ [28].

Overall, two types of coating (TiN and TiN-TiCN, referred as TiCN) were deposited on each of the two substrate types, as presented in

Table 1. Produced sample types.

Substrate	Coating Type	Coated Sample Label
WC-5Co	TiN	WC-5Co-TiN
WC-5Co	TiN-TiCN	WC-5Co-TiCN
WC-15Co	TiN	WC-15Co-TiN
WC-15Co	TiN-TiCN	WC-15Co-TiCN

. Five samples were produced for each of the substrate/coating systems.

2.2. Sample Characterization Methods

2.2.1. Substrate Characterization

A comparative method of density determination was conducted on the sintered WC-Co substrates in accordance with the standard HRN EN ISO 3369:2011 [30]. Magnetic saturation (MS) assessments were conducted to assess the existence of eta η-phase and unbound carbon, employing a sigmameter produced by Setaram Instrumentation (Caluire, France), model D6025, following the guidelines outlined in DIN ISO 3326:2013 [31].

Carbide grain size was determined by measuring the coercivity (H_C) on a Koerzimat 1.096, manufactured by Förster, Reutlingen, Germany.

Surface roughness tests were performed on both as-sintered and polished surfaces using a 3D micro-coordinate system InfiniteFocus XL200 G5 manufactured by Bruker Alicon (Mannheim, Germany). This is important due to the absence of metallographically prepared surfaces on industrially manufactured hardmetal parts utilized in exploitation. The presence of a polished surface is crucial as it facilitates the execution of scratch testing and nanohardness measurement. Considering that surface roughness parameters S_a, S_q, S_z cover a significant surface area, the experiments were conducted on individual samples from each series without repetition.

Hardness testing was carried out by an Indentec reference hardness tester, model 5030 TKV (Indentec, Brierley Hill, UK), utilizing the Vickers technique with a load of 30 kg-force or 294.2 Newtons (HV30), following the guidelines outlined in the standard HRN EN ISO 6507-1:2023 [32]. Ten Vickers hardness measurements were conducted on each substrate.

Instrumented nanoindentation was conducted to determine the value of the reduced (effective) modulus of elasticity (E_r), while the actual modulus of elasticity of the sintered sample (E) was calculated according to Equation (1) [33]:

$$2/E_r=\left(1-{\nu }^2\right)/E+\left(1-{\nu }_i^2\right)/E_i$$

(1)

where ν and E represent the Poisson’s ratio and elastic modulus of the substrate and ν_i and E_i of the indenter, respectively. Nanoindentation measurements were conducted using a maximum force of 1500 mN.

The fracture toughness of the sintered samples was assessed utilizing the Vickers indentation technique by the Palmqvist method, which involves measuring the lengths of cracks l originating from the tips of Vickers indenter impressions, as illustrated in Figure 1.

Measurements were conducted with three repetitions for both groups of samples. The fracture toughness values according to Palmqvist, K_Ic, were determined using Equation (2):

$$K_{Ic}=A_m·\sqrt{HV}·\sqrt{W_G}$$

(2)

where A_m is a constant with a value of 0.0028; HV is Vickers hardness (GPa); and W_G is the ratio of load during the Vickers test, F (N) to the total crack length, T (m): T = l₁ + l₂ + l₃ + l₄.

Surface analysis was utilized to assess porosity and unbound carbon in the WC-Co substrate, with polished surfaces characterized using an Olympus GX51F-5M optical microscope (Tokyo, Japan). Field emission scanning electron microscopy (FESEM, Ultra 55, Carl Zeiss AG, Jena, Germany) was deployed to determine the distribution and size of structural constituents and detect potential microstructural imperfections.

X-ray diffraction (XRD) analysis of phase constituents (Co, WC) was conducted using a Bruker AXS GmbH (Karlsruhe, Germany) X-ray diffractometer, specifically the D8 Advance model.

Comprehensive details regarding all listed testing procedures performed on the samples of WC-5Co and WC-15Co are extensively documented in the literature [13,27,28].

2.2.2. Coated Samples Characterization

The roughness measurements were conducted on three samples for each type of substrate/coating system at the Fraunhofer IST Institute in Braunschweig, on the Form Talysurf Series 2 device manufactured by Taylor-Hobson GmbH. Surface roughness was measured on the coated polished surface of the sample to obtain information on the arithmetic average roughness height R_a required for microhardness testing.

The coating thickness was determined in accordance with the standard HRN EN 1071-2:2003 [35] using the calotte impression method on a Calotest manufactured by TRIBOtechnik. The method relies on the application of a 25 mm diameter steel ball bearing against the sample surface, which, rotating at a speed of 500 rpm for a duration of 45 s, leads to the abrasion of the coating and the formation of craters.

X-ray diffraction (XRD) analysis was also conducted postcoating to determine the phase constituents, crystallographic structure of the coated layers, and possible microstructural changes induced by coating. The analysis was performed at the Faculty of Chemical Engineering and Technology, University of Zagreb. Samples were examined using a Shimadzu XRD6000 X-ray diffractometer (Shimadzu Corporation, Kyoto, Japan) with CuKα radiation, applying an acceleration voltage of 40 kV and a current of 30 mA within the 2θ range of 2 to 120° with a step size of 0.02° 2θ and a dwell time of 0.6 s.

Microhardness and elastic modulus of the coatings were determined through instrumented indentation testing performed using a Fischerscope instrument by Helmut Fischer GmbH, Sindelfingen, Germany. The testing encompassed 5 indentations made on each sample using an indentation force of 50 mN. This force was chosen to ensure that the maximum indentation depth, h_max, reached approximately 1/10 of the coating thickness while also exceeding 1/20 of the surface roughness parameter R_a.

The wear resistance of both the base material and coatings against dry sliding wear was assessed using the ball-on-flat method on the Oscillating Tribotester device developed by TRIBOtechnik. Friction coefficient values without lubrication were determined by alternately sliding the ball across the sample surface. Testing was performed on uncoated samples and on both coating types on each substrate. To accurately simulate operational conditions, the sample surfaces remained as-sintered without any polishing. The tests were conducted with an aluminum oxide ball as counterpart at a maximum force of 10 N and ball movement speed 30 mm/min with the following parameters: 33.20 min duration, 5 mm oscillation amplitude, and total distance travelled of 60 m.

The adhesion of the coating was evaluated using the Rockwell indentation technique following the guidelines outlined in standard ISO 26443:2023 [36] and scratch testing in accordance with standard HRN EN ISO 20502:2016 [37], as detailed in the references [13,27]. The wear trace profile was obtained after sliding wear testing using a VHX-2000 digital microscope by Keyence Corporation (Osaka, Japan). To determine the wear factor (K), which serves as a measure of a material’s resistance to sliding wear, the section area for each wear trace was calculated, and then the wear factor was computed using Equation (3):

$$K=\frac{A·e}{F·s}$$

(3)

where A—wear track section area, mm²; e—motion amplitude, mm; F—normal force, N; s—sliding distance, m.

To detect potential full penetration through the coating during ball-on-flat testing, the depth of wear traces was measured following friction testing. The depths were determined using a Perthometer S8P (Mahr Perthen, Göttingen, Germany).

3. Results

3.1. Substrate Characterization Results

Theoretical density (ρ_th) was derived by calculating the densities based on component contents. Comparing this theoretical density with the measured density allowed for the determination of the sintered sample’s porosity level. The density measurements conducted after sintering indicate a nonporous structure for both substrate compositions. Moreover, it is conceivable to observe slightly elevated densities compared to theoretical values when the η-phase emerges in the microstructure. In the two-phase region of the WC-Co pseudobinary phase diagram, density values might exhibit slight variations. This phenomenon could also contribute to relative densities surpassing 100%.

Examination of magnetic characteristics showed saturation magnetization, registering 7.97 µTm³/kg for WC-5Co samples and 22.10 µTm³/kg for WC-15Co samples. These observations indicate a favorable sintering environment, which helps prevent the occurrence of microstructural abnormalities, such as the presence of η-phase or unbounded carbon [28].

The assessment of coercive properties of the substrate was conducted to indirectly verify the nanograin size postsintering. The sample containing 5 wt.% Co showed an average coercive force of 51.81 kA/m, while the sample containing 15 wt.% Co exhibited a result of 37.23 kA/m (see

Table 2. Properties of substrate samples [27].

Sample	Theoretical/Measured Density, g/cm3	Magnetic Saturation, µTm³/kg	Relative Magnetic Saturation, %	Coercive Force, kA/m	Grain Size, nm	Surface Roughness Parameters, µm
							Ra	Rq	Rz	Sa	Sq	Sz
WC-5Co	14.91/14.194	7.97	88.61	51.81	<0.2	1*	0.196	0.329	1.554	0.416	0.518	4.555
						2*	0.124	0.152	0.580	0.199	0.834	-
WC-15Co	13.71/13.727	22.10	79.43	37.23	<0.2	1	0.148	0.292	1.361	0.359	0.458	4.544
						2	0.069	0.090	0.411	0.123	0.203	-

^1* as-sintered, ^2* metallographically prepared.

). These measurements of coercive force validate the existence of WC grains sized < 0.2 nm, indicating their presence in the nanoscale range.

Surface roughness parameters R_a and R_z were assessed on sintered samples both pre- and post-metallographic preparation. The recorded profiles underwent filtering employing a Gaussian filter, with a cut-off value of 0.8 mm for R_a results ranging from 0.124 μm to 0.196 μm and a cut-off value of 0.25 mm for R_a of 0.069 μm [27].

By comparing hardness values (refer to

Table 3. Mechanical properties of substrate samples [27].

Sample	Hardness, HV30	Poisson’s Ratio of Hardmetals (ν)	Reduced Modulus of Elasticity (Er), GPa	Modulus of Elasticity (ES), GPa	Fracture Toughness, MPa√m
WC-5Co	2268.3 ± 7.7	0.222	353.0 ± 2.7	554.2	8.34 ± 0.07
WC-15Co	1780.9 ± 3.2	0.231	321.8 ± 2.6	475.6	9.24 ± 0.04

) and taking into account the cobalt content, it becomes apparent that they are consistent with data found in the existing literature [30]. This indirectly supports the classification of the material as a hardmetal with grain sizes ranging from ultrafine to nanoscale.

The results presented indicate a clear decrease in hardness as the cobalt content increases. Additionally, there is minimal variation in the measured hardness values, suggesting a uniform microstructure throughout.

The fracture toughness measurements presented in Figure 2 indicate the formation of exceptionally durable hardmetals.

The measured elastic modulus values are anticipated for this material group with tungsten carbide grain sizes in the nanoscale range [38,39,40]. The results also show a significant decrease in the elastic modulus values as the cobalt binder content increases.

Upon comparing the optical micrographs of samples WC-5Co and WC-15Co with the photomicrographs outlined in HRN EN ISO 4499-4:2016 [41], it was confirmed that there are no microstructural imperfections, such as porosity, carbon defects, and the presence of η-phase. The measurements confirmed the results obtained through density and magnetic saturation measurements.

Figure 3 shows micrographs of the samples WC-5Co and WC-15Co that were obtained with a field emission scanning electron microscope at 5000× magnification.

FESEM micrographs of samples WC-5Co and WC-15Co reveal a homogeneous microstructure without the presence of microstructural irregularities, such as carbide phase clustering and abnormal grain growth. The microstructure consists of very fine WC grains uniformly dispersed in a Co matrix, confirming that the extremely small grain size of the initial powders was retained during the sintering process. The actual size of WC grains was determined using the line intercept method in accordance with HRN EN ISO 4499-2:2020 [42]. FESEM micrographs at a magnification of 20,000 times were employed for this investigation. The average carbide grain sizes were found to be 187.71 ± 1.17 nm for the WC-5Co sample and 191.59 ± 0.82 nm for the WC-15Co sample, confirming grain size in both samples of below 200 nm. These results validate that sintering parameters were properly set to avoid grain growth, as confirmed by coercive force measurement results.

Examination of the samples via XRD analysis revealed the existence of two crystalline phases, a hexagonal-structured (HCP) WC phase and a face-centered cubic (FCC) lattice Co phase, as presented in the literature [27]. These findings reaffirmed the absence of η-phase and unbound carbon content.

3.2. Coated Samples’ Characterization Results

3.2.1. Coating Surface Roughness and Thickness Measurements

Surface roughness was measured on the coated polished surface of the sample to obtain, among others, information on the average surface roughness R_a required for microhardness testing. Low surface roughness negatively affects coating adhesion but is desirable in cases of adhesive wear. The surface roughness values of the coated samples are provided in

Table 4. Coated samples’ roughness measurement results.

Sample	Roughness Parameters, µm		Coating Thickness, µm
	Ra	Rz
WC-5Co-TiN	0.2114 ± 0.0146	1.5174 ± 0.0993	3.05 ± 0.30
WC-15Co-TiN	0.1890 ± 0.0407	2.0255 ± 0.0174	3.14 ± 0.26
WC-5Co-TiCN	0.2076 ± 0.0133	1.5648 ± 0.1819	5.19 ± 0.39
WC-15Co-TiCN	0.1341 ± 0.0052	1.0841 ± 0.0365	5.32 ± 0.18

. By comparing the surface roughness results before and after coating, it is evident that there is no significant increase in the roughness as a result of individual layer deposition.

The thickness of coatings represents the most influential factor characterizing its tribological behavior, significantly affecting coating adhesion. Throughout the coating deposition process, uniform coating thickness on various samples was the aim, with each sample positioned at the same height within the PACVD chamber during coating. The coating thickness values obtained through the Calotest method (Figure 4) indicate that the presence of cobalt content did not have a significant impact on the growth of the individual coatings.

3.2.2. Coated Samples’ XRD Analysis Results

A total of four types of coated samples were examined concerning potential combinations of coating and Co content in the substrate, with diffractograms presented in Figure 5.

The WC-Co hardmetal samples were coated with a uniform layer comprising titanium nitride, as well as alternating layers of titanium nitride and titanium carbide. The dominant phase observed was hexagonal P-6m2 tungsten carbide (WC, ICCD PDF#51-0939), characteristic of the substrate hardmetal. Surface treatments revealed the presence of cubic Fm-m3 titanium nitride (TiN, ICCD PDF#38-1420), indicated by a weak and broader peak at 42° 2θ. Cubic Fm-m3 titanium carbonitride (TiC0.7N0.3, ICDD PDF#42-1489) was observed in traces as well. The titanium nitride, titanium carbide, and titanium carbonitride phases exhibit almost identical crystal structures, resulting in very similar diffraction patterns. Consequently, it is challenging to definitively identify them. XRD analysis verified that the coating did not induce the formation of carbon defects or η-phase in the substrates’ surface layers.

3.2.3. Mechanical Properties of Coated Samples

The microhardness measurements revealed consistent values for substrates sharing the same coating type, with approximately 2200 HV 0.005 for TiN-coated samples and 3200 HV 0.005 for those with TiCN coating,

Table 5. Nanoindentation test results of coated samples.

Sample	Microhardness, HV 0.005	Indentation Modulus of Elasticity EIT, GPa	Maximum Indentation Depth, µm
WC-5Co-TiN	2245.6 ± 36.9	345.6 ± 32.6	0.3158
WC-15Co-TiN	2232.2 ± 64.8	348.4 ± 27.7	0.3161
WC-5Co-TiCN	3256.6 ± 46.8	401.0 ± 12.2	0.2645
WC-15Co-TiCN	3274.2 ± 34.2	399.6 ± 12.7	0.2546

. As anticipated, the harder TiCN-coated samples exhibited lower indentation depths, accordingly. The observed indentation depths were notably smaller than the thickness of the coatings, indicating a minimal influence of the substrate on the coating’s hardness. Contrary to expectations, the variability in measured hardness values was not large despite the application of low indentation load.

The determination of the indentation’s Young’s modulus (E_IT) of the coatings was obtained by analyzing the force (F)—indentation depth (h) curves obtained through nanoindentation testing, Table 5.

3.2.4. Coating Adhesion Test Results

Adhesion of the coating, a crucial property demanded for tribological protection, was assessed using the Rockwell indentation test and scratch testing. Adhesion evaluation involved testing of one sample for each tested system of base material/coating, with three imprints made on each sample. An examination of cracks and delamination in the coating, resulting from localized stresses and deformation at the indentation site, was visually conducted (

Table 6. Rockwell coating adhesion test results.

Sample	Adhesion Class	Rockwell Test Indent
WC-5Co-TiN	HF-5
WC-15Co-TiN	HF-5
WC-5Co-TiCN	HF-3
WC-15Co-TiCN	HF-3

) and compared with the various classes according to VDI 3198 [43].

The TiCN coating demonstrated better adhesion, classified as HF-3, regardless of the substrate type, with no visible presence of delamination or with only minor instances characterized by a greater number of cracks surrounding the impression. Samples with TiN coating were classified as HF-5 adhesion class, indicating predominantly satisfactory adhesion with certain delamination of the coating. Nevertheless, the presence of delaminated areas suggests that the TiN coating may struggle to withstand potential severe operating conditions.

The results obtained by the Rockwell test were also confirmed by the scratch test results,

Table 7. Scratch test results of coated samples.

Sample Type	Critical Coating Delamination Force, N
	Lc1	Lc2	Lc3
WC-5Co-TiN	-	22.49 ± 5.13	26.19 ± 6.36
WC-15Co-TiN	-	25.65 ± 4.91	29.46 ± 6.47
WC-5Co-TiCN	-	40.36 ± 2.74	48.09 ± 2.33
WC-15Co-TiCN	-	34.58 ± 3.83	47.08 ± 3.35

. Data for the WC-5Co-TiN sample show a significantly lower TiN critical force of coating delamination (L_c2) of 22.49 N. The higher-quality TiCN coating demonstrated an almost doubled value of 40.36 N for the same substrate material. In contrast, a higher force Lc3 of approximately 48 N and 47 N was necessary to penetrate the TiCN coating of WC-5Co-TiCN and WC-15Co-TiCN samples, respectively. The single-layer TiN coating applied to samples of WC-5Co-TiN and WC-15Co-TiN failed to meet fundamental adhesion criteria. The TiN coating exhibited slightly improved performance on samples with higher cobalt content. None of the tested samples showed any cracks in the coating characteristic of force L_c1.

3.2.5. Tribological Properties Testing Results

The tribological properties of both the base material and coatings in terms of their resistance to dry sliding wear were examined using the ball-on-flat method. Figure 6 and Figure 7 present the mean friction coefficient values for the tested base material/coating systems.

Table 8. Mean values of volume loss and friction coefficient determined by dry sliding wear.

Sample	Mean Friction Coefficient, µ	Volume Loss ∆V, mm3
WC-5Co	0.333	0.0030
WC-15Co	0.323	0.0043
WC-5Co-TiN	0.137	0.0031
WC-15Co-TiN	0.137	0.0030
WC-5Co-TiCN	0.135	0.0025
WC-15Co-TiCN	0.135	0.0025

presents the average values of the friction coefficient and volume loss determined by this method. A comparison of the measured values clearly indicates that higher friction coefficients are exhibited by uncoated samples, while coating with TiN and TiCN contributes to the reduction of the friction coefficient. The variation in friction coefficient observed in TiCN coating can be partially attributed to the ball passing through different layers of multilayer coatings.

The test findings align with the anticipated outcomes for this substrate and the coatings applied. A harder surface typically correlates with reduced volume loss, leading to a lower wear factor. Figure 8 presents calculated wear factors for each sample type.

When dry sliding wear factors of the uncoated substrates are compared, higher factor values and volume loss are evident for samples with higher Co content, which are generally softer. Nevertheless, after coating, there is a noticeable decrease in dry sliding wear across all substrate types. This reduction can be attributed to the formation of a harder surface layer, resulting in less volume loss. Upon analyzing the wear factors for the coated samples, we found no significant differences regardless of the substrate Co content. Lower wear factor was detected for samples with TiCN coating. With all mentioned, stable behavior of the coatings under dry sliding wear conditions was confirmed.

4. Conclusions

In pursuit of developing nanograin hardmetals and innovative surface coatings using PACVD technology, investigations were undertaken to examine the characteristics of nanostructured hardmetal substrates, newly developed surface coatings, and the resulting substrate/coating systems.

Based on the conducted research, the following conclusions can be drawn:

(i)

The PACVD process proved very effective in generating two coating systems on a different WC-Co carbide substrate: a monolayer TiN coating and a multilayer gradient TiCN coating (alternating TiN and TiCN layers).

(ii)

Analysis revealed no significant deviation in the roughness values among the TiN and TiCN coatings deposited on substrates with different compositions, indicating that the Co content does not have a significant impact on layer growth.

(iii)

Measurements of coating thickness show uniform values for individual coating type. TiCN coating exhibited higher thickness values of 5.19 ± 0.39 µm and 5.32 ± 0.18 µm for WC-5Co and WC-15Co compared to the single-layer TiN coating with layer thickness of 3.05 ± 0.30 µm and 3.14 ± 0.26 µm, respectively.

(iv)

The Rockwell test utilized as an indirect method for determining coating/substrate adhesion showed that the single-layer TiN coating did not meet the essential adhesion criteria on both tested substrates. This was confirmed by the scratch test results, which measured significantly higher values of the critical force of delamination and penetration in the case of TiCN coating.

(v)

Application of PACVD thin, hard layers can result in a significant reduction of friction coefficients by almost 60% compared to the uncoated substrate, regardless of its composition. Wear factor also confirms significant reduction in wear after coating, especially for a softer substrate with higher binder content.

This comprehensive study resulted in important data on mechanical and tribological properties of chosen hardmetals/coating systems obtained by PACVD technology, thus offering valuable insights for further research and practical implementation of coated WC-Co hardmetals in relevant industries such as the cutting tool industry, mining, and construction.