*4.2. Tool Coatings*

Two of the most prominent types of coating methods are the Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD). In the case of the latter, which uses higher temperatures on the process, the coating is synthesized from a mixture of different gases, depending on the type of coating being deposited [94]. On the other hand, the PVD technique intensively improves the wear resistance of different tools, effectively increasing their lifespan [95].

Studies show that the correct application of a coating is more effective than an uncoated tool in drilling multi-materials [96]; however, this is only true for higher feed rates and cutting lengths, severe conditions, as coated tools possess a much higher cost, so these are only applied when strictly necessary. Coated carbide tools such as PVD coated Boron-Aluminum-Nitride (BAM) tools have a better performance compared with uncoated tools in terms of tool wear at severe drilling conditions. In spite of this, prolonged tool life came with the price of rougher finished surfaces due to roughness of the BAM coating [86].

Figure 14 shows three different types of coatings which are common for rotary tooling such as drilling, being two monolayer (TiN and TiAlN) and one multilayer (AlTiN/TiAlN). The latter combines two layers for better heat and wear resistance [87]. From a study performed with these three coatings, the TiAlN drills produced the largest thrust and cutting forces, with the AlTiN/TiAlN resulting in the lowest values. Additionally, this coating presented a better self-lubricating effect due to its multilayer structure, which also increases its hardness, resulting in a better performance achieved during drilling [97].

**Figure 14.** Drilling tools with different coatings (adapted from [87]).

Diamond CVD coatings are many times chosen as well, due to their enhanced characteristics when compared to the remaining options, although sometimes they experience some lack of adhesion. Nevertheless, when these problems are overcome, diamond coatings provide a longer lifetime when put side by side with other coating materials [98]. Its high thermal conductivity contributes to a temperature decrease in the cutting tool surface, and a low thermal expansion coefficient mismatch allows obtaining a good adhesion between the CVD diamond film and the ceramic substrate, as a result of a lower interface residual stress at room temperature. It also contributes to an enhanced surface hardness, which leads to a decrease in the tool wear [99]. Diamond-like carbon (DLC) coatings are also often used in the literature as an alternative to a diamond film [100].
