A Simulation Study on the Effect of Filament Spacing on the Temperature Field Uniformity of an HFCVD System

Abstract

Hot-filament chemical vapor deposition (HFCVD) has become the most widely used ways of preparing diamond film-coated tools due to the simple equipment used, its convenient operation, and its low cost. In the production process of an actual factory, a large number of coated tools need to be prepared in batches. Factors such as the hot-filament arrangement often affect the uniformity of coating on tools, making the performance of the tools prepared in the same batch unstable. This article uses ANSYS R15.0 software software in the context of computational fluid dynamics (CFD) to calculate the temperature field in the HFCVD system, and study the effect of filament spacing on the uniformity of the temperature field of the surface of the substrate. It was found that when the distance between filaments was 14 mm, 10 mm, 10 mm, 8 mm, 8 mm, the temperature field on the surface of the substrate was the most uniform. The experiments are consistent with the results of the simulation, indicating that simulation research has practical significance.

1. Introduction

Due to its extremely high hardness, elastic modulus, thermal conductivity, and extremely low coefficient of friction, diamond has been widely used in the field of machining. It is often used to make tools and wear-resistant parts. It can also increase the life of a tool and improve the quality of the surface of a machine; furthermore, it can reduce the roughness of the machined surface [1,2]. Due to the high strength and toughness of cemented carbide, it can be used as a matrix material, and the deposition of diamond on its surface can improve the wear resistance of the tool, reducing friction, thereby greatly improving the life of the tool [3,4]. The hot-filament chemical vapor deposition (HFCVD) method for preparing diamond coatings is simple and low-cost, which is very suitable for the industrial production of large-area diamond coatings [5,6,7]. However, when depositing diamond films on a large area, the uniformity of the coating is often difficult to control. Therefore, in addition to ensuring the excellent performance of the diamond coating, the uniformity of the diamond coating is also another important factor limiting its industrial production. Therefore, to deposit high-quality diamond coatings, it is necessary to change the deposition conditions to ensure the physical field is near the substrate and the hot filament is stable and uniform [8,9,10]. Due to the complex and numerous factors affecting the temperature field during the deposition process, many scholars have studied improving the uniformity of the temperature field on the basis of computer modeling and simulation [11,12]. The study found that the thermal radiation of the hot filament is the main factor affecting the distribution of the temperature on the surface of the substrate. In addition, the temperature of the substrate is affected by the temperature, diameter, and number of the hot filament. And the relationship between them is one of a positive correlation [13,14,15,16]. However, the influence of filament spacing on the uniformity of the temperature field is not mentioned specifically. Therefore, based on computational fluid dynamics (CFD), this paper solves the temperature field distribution of an HFCVD system, analyzes the influence of filament spacing (equal spacing and unequal spacing) on the temperature field distribution of the system, and obtains the filament spacing of the HFCVD system with the best temperature field; this can guide the structural design of the actual CVD deposition system to improve the quality and uniformity of diamond films.

2. Geometric Model and Calculation Method

2.1. Geometric Models

This article selected the SolidWorks 2024 software to establish a model. SolidWorks is a 3D CAD design software widely used in product design, simulation, manufacturing, and engineering management, supporting designers in the complete development process from concept to product. In order to improve calculation efficiency, a quarter of the HFCVD reaction ventricular cavity was used as the research object, and two symmetrical planes were set. The modeling results are shown in Figure 1.

The hot filament was tungsten, with a total of 10 roots. The filament axis was the Z axis, arranged in the direction of the X axis. The substrate was YG6 with dimensions of 16 × 16 × 4.5 mm. The workbench material was molybdenum with specifications of φ 55 × 30 mm. Using previous experimental research, the deposition parameters were 2% for carbon source concentration, 3 KPa for deposition pressure, and 5 mm for the distance between the filaments and substrate.

2.2. Calculating Parameters

The various parameters used in calculation are shown in

Table 1. Parameters of simulations.

Unit	Material	Thermal Conductivity (λ)/(W/m·K)	Specific Heat Capacity (c)/(J/kg·°C)	Density (ρ)/(kg/m3)	Viscosity Coefficient (λ)/(w/m·K)
reaction gas	H2	0.1289	7243	0.0899	2.41 × 10−4
filament	W	174	132	19300	-
substrate	YG6	92	188	14600	-
substrate table	Mo	138	251	10240	-

.

The calculation started after all the parameters were set; the number of iterations was about 150–170. The convergence curve is shown in Figure 2.

2.3. Calculation Method

This article takes fluid mechanics as its theoretical basis [17,18]; the ANSYS R15.0 software used the finite volume method [19,20]. This method was characterized by its clear physical meaning and a fast solution of discrete equations. The heat exchange effect of an HFCVD system is very complicated, and three heat transfer methods (conduction, convection, and radiation) coexist. This article selected the relevant parameters to set according to the needs of temperature field simulation, and made appropriate assumptions [21,22,23]. We assumed that the heat exchange in the HFCVD system involved only the thermal radiation from the filament to the substrate and workbench, the radiation from the filament to the gas, and the convective heat transfer inside the gas. The ANSYS R15.0 software contains 5 radiation models [24,25,26]. We chose the DTRM model as a thermal radiation model to calculate the temperature field. The DTRM model is a discrete radiative heat transfer model, which assumes that the radiation leaving surface elements at multiple angles within a certain range can be approximated as a ray, and an increase in the density of these rays will improve the calculation accuracy.

3. Results and Analysis of Simulation

3.1. The Effect of Different Spacing of Filaments on the Temperature Field

Figure 3 shows the distribution of the matrix temperature field under different distances between filaments. The node temperatures in the X and Z directions on the centerline of the substrate surface were extracted, with the Z direction parallel to the filament direction and the X direction perpendicular to the filament direction. Moreover, due to the symmetrical relationship and in order to save calculation time in the modeling process, only 1/4 of the model was built. The positive direction of the X coordinate axis and the negative direction data of the Z coordinate axis were selected and drawn. The distribution curves of temperature are shown in Figure 4 and Figure 5.

From Figure 3, the influence of a different distance between filaments, T_W, on the temperature distribution of the substrate can be analyzed. When the T_W was small, the temperature presented a trend of being “high in the middle and low on both sides”, the difference was obvious, and the temperature distribution was extremely uneven. Moreover, the smaller the T_W, the higher the temperature on the surface of the substrate and the greater the difference. With the increase in T_W, the distribution of hot filament over the whole substrate tended to disperse, and it was found that when the T_W increased to 14 mm, the temperature of the middle substrate would be slightly lower than that of the surrounding substrates. The main way to transfer heat was radiation heat transfer, and the difference in temperature on the surface of substrate was no more than 100 °C. When the hot filament was centrally distributed above the substrate, the concentration of radiation sources would produce concentrated heat radiation. The hot filament would not only directly radiate the heat to the substrate, but also to the surrounding reaction gas. The gas would transfer a small amount of heat to the substrate or take away the heat on the surface of the substrate through convective heat exchange. Therefore, when the T_W was small, the convective heat transfer of the airflow and the concentrated heat radiation of the heating filament made the temperature of the substrate in the middle higher and in the surrounding parts lower. With the increase in T_W, the heating filament had good heat radiation for each substrate, and the temperature distribution of the substrate gradually tended to be uniform, and the temperature difference between the middle substrate and the two substrates would also change a small amount. However, it can be seen from the cloud diagram that when T_W was increased to 14 mm, the temperature of the middle of the substrate fluctuated greatly, and the temperature of the middle part was significantly lower than that of other places. The reason may be related to the convective heat transfer of the airflow. The airflow velocity in the X direction of the center line fluctuates, but the center velocity is the lowest, so the convective heat transfer is the smallest. In the scatter diagram, values of 0–0.008 m and 0.013–0.029 m in the X direction were used to observe the temperature on the surface of the substrate, and values of −0.008–0 m and −0.029–0.013 m in the Z direction were used to observe the temperature on the surface of substrate. It can be seen from Figure 4 and Figure 5 that the temperature of the center of the substrate in the X direction decreases with the increase in T_W, but the temperature of the substrate on both sides showed an overall downward trend with the increase in T_W. Furthermore, the temperature of the substrate when the T_W was 8 mm was lower than that when it was 10 mm. The difference between the temperature of the substrate when the T_W was 12 mm and 14 mm was small. The fluctuation in temperature in the negative direction of the Z axis was very small, and the distribution was relatively uniform. As the distance between the hot filament increased, the temperature of the substrate in the Z axis decreased.

In order to intuitively judge the uniformity of the temperature field on the surface of the substrate under different values of T_W, the standard deviations of the temperature values in the positive direction of the X axis and the negative direction of the Z axis are compared, as shown in

Table 2. Standard deviation of X and Z direction temperature under different T_W.

TW/mm	Standard Deviation of Temperature in X Direction/°C	Standard Deviation of Temperature in Z Direction/°C
8	35.843	5.034
10	13.933	6.494
12	15.318	5.866
14	16.149	7.535

.

Generally speaking, the temperature of substrate in the Z direction is more uniform than that in the X direction. The temperature uniformity in the Z direction is not much different, while that in the X direction is much worse. In contrast, when the T_W was 10 mm, the distribution of the temperature field was more uniform, which is mainly related to the arrangement of the filament and the gas flow path.

3.2. Influence of Different Spacing of Filaments with Different Spacing on Temperature Field

Considering the influence of the substrate worktable diameter and the filament arrangement on substrate temperature, we selected five sets of filament arrangements. The symmetrical side of the filament arrangement was denoted by the distance from the middle outwards as Tw1, Tw2, Tw3, Tw4, and Tw5. The first set, labeled 1#, features distances between filaments of Tw1 = 14 mm, Tw2 = 12 mm, Tw3 = 10 mm, Tw4 = 10 mm, and Tw5 = 8 mm; the second set, labeled 2#, has distances between filaments of Tw1 = 14 mm, Tw2 = 12 mm, Tw3 = 12 mm, Tw4 = 10 mm, and Tw5 = 8 mm; the third set, labeled 3#, features distances between filaments of Tw1 = 14 mm, Tw2 = 10 mm, Tw3 = 10 mm, Tw4 = 8 mm, and Tw5 = 8 mm; the fourth set, labeled 4#, has distances between filaments of Tw1 = 12 mm, Tw2 = 10 mm, Tw3 = 10 mm, Tw4 = 8 mm, and Tw5 = 8 mm; the fifth set, labeled 5#, features distances between filaments of Tw1 = 10 mm, Tw2 = 10 mm, Tw3 = 8 mm, Tw4 = 8 mm, and Tw5 = 8 mm. Figure 6 depicts the temperature distribution cloud map of the substrate with filaments arranged at varying unequal distances. Similarly, temperature values were selected from nodes on the positive X axis and negative Z axis, and scatter plots of the temperature distribution were created for comparative analysis, as illustrated in Figure 7 and Figure 8.

It can be seen from the figure that in the three cases where the center spacing was 14 mm, due to the larger distance between the hot filaments in the middle and the smaller distance between the hot filaments on the two sides, the radiation intensity on the middle substrate was significantly lower than that of the two substrates. The temperature of the substrate in the middle was be slightly lower than that on both sides, and the temperature of the substrate directly under the filament was also slightly higher than the temperature of the surrounding substrates. Numbers 1 and 2 were the most obvious, which showed the “hot-filament effect”. The distribution was the most uniform in the case of number 3. The temperature of the middle part of the intermediate substrate was still lower than the temperature of the surrounding substrate. Therefore, number 4 and number 5 were selected to arrange the hot filaments to shorten the distance between the filaments, but the effect was not ideal. Therefore, under the currently situation, number 3 could make the temperature of the substrate the most uniform. It can also be seen from the scatter diagram that in the case of number 3, the substrate not only receives the most heat radiation energy, but also the surface temperature of the substrate is the most uniform regardless of whether it is in the X direction or the Z direction.

Table 3. Standard deviations of temperature in the X and Z directions with unequal spacing of hot filaments.

Sample	Standard Deviation of Temperature in X Direction/(°C)	Standard Deviation of Temperature in Z Direction/(°C)
1#	16.143	7.584
2#	16.200	4.379
3#	11.204	3.850
4#	22.403	10.306
5#	14.438	4.217

showed the standard deviation of the temperature values in the positive direction of the X axis and in the negative direction of the Z axis in different cases of unequally spaced filament arrangement. The uniformity of the temperature could be directly judged by the magnitude of the standard deviation.

It can be seen from Table 3 that the standard deviation of the temperature values in the X and Y directions of the #3 unequally spaced hot-filament arrangement was the smallest; that is to say, the temperature field on the surface of the substrate was the most uniform. Then comes the #5 hot-filament arrangement. Both #3 and #5 had more a uniform temperature field on the surface of the substrate than those which were arranged at equal intervals.

4. Experimental Verification of Simulation Results

4.1. Experiment

Considering the symmetry of the filament arrangement, the four substrates were selected for deposition experiments. After “three-step method” pre-processing [27], the substrates were placed on the workbench, as is shown in Figure 9. The parameters utilized were as follows: tungsten filament (Φ1.0 mm, 4 wires), acetone as the carbon source, and hydrogen as the auxiliary gas. The deposition was initiated once the reactor had been completely evacuated and the filament had carbonized. The initial vacuum must be lower than 4 Pa. We chose the best filament arrangement of the uniformity of the temperature field; that is, the distance between the filament was 14 mm, 10 mm, 10 mm, 8 mm, and 8 mm, and the substrate spacing was 8 mm. The deposition parameters are shown in

Table 4. Parameters of deposition.

Parameters	Filament Carbonization Process	Coating Deposition Process
gas flow (mL/min)	1000	1000
hydrocarbon ratio/%	4	2
filament temperature/°C	2100 ± 100	2300 ± 100
substrate temperature (°C)	—	700~850
filament spacing Tw	3#	3#
filament/substrate distance/mm	—	5
substrate spacing	8 mm	8 mm
background vacuum/Pa	≤4	≤4
filament power/kW	8	10
reaction pressure /kPa	6	3
reaction time/min	120	240

.

We used HBRVU-1875 Blovi Optical Hardness Meat (Suzhou Kangyang Automation Co., Ltd, Suzhou, China) to make an indentation on the sample and used the indentation to characterize the adhesion of the coating, the load was 588 N, and the loading time was 10 s. The surface morphology of indentation was characterized using a scanning electron microscope (SEM-S-3400,Shenzhen Xinyichuang Technology Co., Ltd, Shenzhen, China) with an operating voltage of 20 kV.

4.2. Experimental Results and Analysis

Figure 10 shows the SEM images of indentation of diamond coatings with different positions. It can be seen from Figure 10 that the adhesion of the four coatings was high, and there were no obvious cracks and coating peeling. Among them, the indentation of #4 was obvious, but it was not much different from the other three samples. This might be because the position was relatively close to the edge and the deposition temperature decreased slightly. The indentation of the four samples was similar, indicating that the quality of the diamond coating was similar, the uniformity of the temperature field was good, and a large number of productions could be performed. In addition, defects, interfaces, etc., can all affect the adhesion of the coating. The utilization of energetic ions bombarding the coating during growth can efficiently promote adhesion by increasing the density of the surface and interface structure. High-power impulse magnetron sputtering (HiPIMS) and deep-oscillation magnetron sputtering (DOMS) would be worth studying in depositions of high-performance coatings with a high surface integrity, dense microstructure, low residual stress, uniform thickness, and high adhesion [28].

5. Conclusions

(1)

When the distance between filaments was equal and the number was small, the temperature on the surface of the substrate showed a phenomenon of being “high in the middle and low on both sides”. With the increase in T_W, the distribution of the filaments over the entire substrate tended to be dispersed; this phenomenon disappeared slowly. When the T_W increased to 14 mm, the temperature of the intermediate substrate was be slightly lower than the temperature of the surrounding substrate.

(2)

When the filaments were equally spaced and the distance was 10 mm, the temperature field on the surface of the substrate was relatively uniform. When the filaments were unequally spaced and the spacing was 14 mm, 10 mm, 10 mm, 8 mm, 8 mm, and 8 mm, the temperature field on the surface of the substrate was the most uniform, and this arrangement was the best.

(3)

The experimental results were consistent with the simulation results. When the filament spacing was 14 mm, 10 mm, 10 mm, 8 mm, 8 mm, and 8 mm, the coating adhesion was high, and the product stability was high, this allowed for a large quantity of production.