Coatings of TiN have been deposited by thermal CVD between 850 and 1050 °C and characterized with respect to their microstructure as well as mechanical and tribological properties. X-ray diffraction (XRD) analysis yields the formation of textured films, particularly at low deposition temperatures. It is shown that the coating hardness changes with the texture, where highly (2 1 1) oriented films exhibit a lower hardness compared to non-textured coatings. The wear resistance has been determined by dry sliding experiments against Al2O3 balls at 25 °C on both as-deposited and polished samples. Independent on the surface roughness, the smaller grain size of the textured films causes a higher toughness and results in a better wear resistance.

The effect of deposition temperature on microstructure and properties of thermal CVD TiN coatings

TiCxN1-x base layers with Al2O3 top layers, both grown by chemical vapour deposition (CVD), are state-of-the-art in metal cutting with cemented carbide inserts. In order to influence the microstructure and properties of the base layer, five different TiCxN1-x coatings were grown by medium-temperature CVD with increasing CO fractions in the TiCl4–CH3CN–H2–N2–CO feed gas using an industrial-scale low-pressure CVD system. With the CO fraction in the feed gas rising up to 2.2 vol.%, oxygen contents of 2.2. at.% could be detected, which are distributed homogeneously over the coating thickness. The originally equiaxed TiCxN1-x grains with preferred {110} orientation obtained without CO addition change to randomly distributed plate-like grains, while the grain size decreases to the sub-micron range. Also the surface roughness decreases with rising CO addition. The incorporation of oxygen leads to a homogenous distribution of dislocations within the TiCxN1-x grains and to an increasing density of twin boundaries. The residual tensile stress shows a minimum of 200 ± 47 MPa, while the hardness increases to 29.1 ± 1.3 GPa and the elastic modulus reaches a maximum 569 ± 29 GPa for CO fractions of 1.5 vol.% in the feed gas.

CO addition in low-pressure chemical vapour deposition of medium-temperature TiCxN1-x based hard coatings

Wear resistance of the prior-art Ti(C,N) layers can be considerably enhanced by optimising the grain size and microstructure. Considerably better wear resistance in, for example in many carbon steels, can be obtained by modifying the grain size and morphology of prior art MTCVD Ti(C,N) coatings. The improved coating is composed of small columnar crystals. Doping by using CO, CO2, ZrCl4, HfCl4 and AlCl3 or combinations of these can ensure the control of the grain size and shape. Doping has to be controlled carefully in order maintain the columnar structure and also in order to avoid nanograined structures and oxidisation. The preferred grain size should be in the sub-micron region with the grain width of 30 to 300 nm. The length to width ratio should be more than 5, preferably more than 10 and the coating should exhibit a strong preferred growth orientation along 422 or 331. The XRD line broadening should be weak.

Layer with controlled grain size and morphology for enhanced wear resistance

To improve the cutting performance, a modified MT-TiCxN1 − x coating (x = 0.63) that adheres well to carbide substrates was developed by introducing C2H4 gas to a TiCl4–CH3CN–H2 precursor system on a TiN base layer during moderate temperature low pressure CVD. Glow-discharge optical emission spectroscopy and X-ray diffraction analyses reveal an increase in the incorporated carbon content. Electron backscatter diffraction analysis shows a highly preferred orientation [(422) plane] with a very fine columnar grain size of 0.17 ± 0.06 μm. The increase in hardness and the decrease in friction coefficient for cutting AISI: 51200 and AISI: 316 at room temperature and 600 °C with the modified MT-TiCxN1 − x coating result in a 30% longer tool life for continuous cutting of carbon steel (AISI: 1055) and alloyed steel (AISI: 4137) and a 60% longer tool life for stainless steel (AISI: 316). The modified MT-TiCxN1 − x coating resistance to failure was determined by four-groove intermittent cutting of AISI: 4137, and it shows a 30% increase in the average number of impacts compared with a conventional MT-TiCxN1 − x coating.

Improvement of mechanical properties and cutting performance of modified MT-TiCxN1 − x coating by moderate temperature chemical vapor deposition

Typically, hard coatings are produced via condensation from the vapor phase. In chemical vapor deposition (CVD) gaseous precursor vapors at elevated temperatures react and form the desired chemical compound of the coating. Since high temperatures are necessary to deposit most of these hard coatings, substrate materials are mainly limited to cemented carbides. CVD processes are suitable for coating parts with complex geometries, can produce relatively thick coatings, and characteristically yield crystalline oxide coatings. Physical vapor deposition (PVD) processes generate the vapor from a solid source by physical methods and are generally conducted at lower temperatures, enabling the use of metallic substrate materials. Ionization of the reactant species and low deposition temperatures permits the deposition of metastable coatings, far from the thermodynamic equilibrium, a major reason why PVD is a flexible process that can produce a broad variety of coatings.

This chapter addresses the history of CVD and PVD coatings, their technological aspects, coating architectures, and finally their range of application and why in some regions one technology dominates and in others both are competitive.

1.16 – Coating Applications for Cutting Tools

Chemical vapor deposition of Ti(C,N) coatings from the TiCl4–CH3CN–N2–H2 system on cemented carbide substrates was studied. The morphology and grain size of the coatings were modified using carbon monoxide (CO). Transmission electron microscopy confirmed that grain refinement of the Ti(C,N) coatings could be obtained by means of CO doping and nanocrystalline coatings were obtained at CO levels exceeding 6%. CO doping resulted in the incorporation of oxygen in the structure, but no segregation of oxygen to grain boundaries was observed. The coatings appeared to be of homogeneous composition even at the highest CO levels. Both improved coating hardness and surface quality were obtained with decreasing grain size. However, the coatings exhibited clearly lower crater wear resistance in the nanograined region. This was explained by an increased tendency for grain-boundary sliding in the nanocrystalline coatings leading to more pronounced plastic deformation.

A. Larsson

Papers

Deposition, microstructure, and properties of nanocrystalline Ti(C,O,N) coatings