ICMCTF2000 Session B3-2: Structure and Properties of Hard Coatings

Nanocomposite thin films consisting of nanosized solid solutions or nanosized polycrystalline materials embedded in various amorphous matrix materials provide a great potential as hard coatings for future mechanical devices. Films resulting from additions of Si to TiN matrix, using a closed field unbalanced magnetron sputtering system, were prepared and characterised in this paper.

Physical and structural properties such as composition, texture and grain size, and mechanical properties such as hardness, Young´s modulus, residual stresses and adhesion were analysed. Nanoindentation tests and scratch tests were carried out for the mechanical characterisation. The atomic composition of the samples was obtained by Rutherford Backscattering Spectrometry (RBS). X-ray diffraction (XRD) experiments were used in order to obtain texture and grain size. The deflection method was used to evaluate the macroresidual stress states.

All properties will be characterised and discussed as a function of the Si content in the Ti-Si-N matrix, deposition rate, bias voltage, working pressure and distance from target and several relations will be made regarding important parameters such as texture, grain sizes and stress states. Regarding the preliminary results, the samples show hardness values as high as 45 GPa. Almost all samples showed high critical adhesion loads for total failure, generally higher than 80 N. The texture, as a function of Si content changes from a (111) orientation to a (200) orientation. Relations within these results, together with those of residual stresses will be made.

Ceramic coatings have been widely used in cutting tools and various machine parts. Even though high strengths have been obtained in most ceramic coatings, it has also been shown that ceramic coatings undergo extensive plastic deformation during scratch and wear tests. It is essential to understand the plastic flow and related friction and wear behaviour. Reciprocating multipass wear tests have been carried out on various CVD coatings. Deformation and failure mechanisms were identified based on high resolution microstructural examination through cross-sectional fracture surfaces

Obvious plastic flow was observed on the surface of CVD ceramic coatings in the first sliding, due to the extremely high contact pressure developed on the contact asperities. However, shake down may quickly be reached after several subsequent traverses. In further repeated traverses, the plastic-elastic flow accumulates residual strain energy may reach certain level where stress induced cracking, microbuckling, and microflaking will occur along the elastic-plastic interfaces behind the indenter. The new surface will appear after the detachment of the heavily strained plate-like wear debris. The repeated sliding allows the process- “plastic flow of asperities - flatten the surface and shake down - microbuckling and detachment of strained layer to continue until the coating is totally worn out.

Multilayered PVD coatings have shown enhanced mechanical and tribological properties as compared to homogeneous coatings. In this work, multilayered PVD TiN/TaN coatings deposited on cemented carbide tools are presented and compared with homogeneous TiN and TaN. TiN/TaN has in earlier investigations shown high abrasive and erosive wear resistance as well as good properties in milling austenitic stainless steel. The purpose of this work is to increase the understanding of the wear and failure mechanisms during milling. For this reason milling to a certain cutting distance, facilitating a direct comparison of the coatings, has been performed.

It was found that by multilayering TiN with TaN, a coating suitable for tools used in milling austenitic stainless steel can be obtained. The reason is that it is possible to take advantage of the good properties of TaN, such as low thermal conductivity and high hot hardness, as well as the high toughness obtained through crack deflection in lamella interfaces.

Keywords: PVD, TiN, TaN, milling, wear

Thin film coating techniques have become more and more important in the deposition of wear resistant coatings for tribological applications. Thin film coatings have proved to be remarkably effective for the increment of lifetime for cutting tools or moulds.

For improving the wear performance and toughness of thin films, ternary or multiphase coatings become attracting as they provide the possibility of tailoring material properties by synthesizing the coating with a suitable combination of different phases. In the ternary family of transitional boron nitrides, Ti-B-N and Zr-B-N are developed and well studied. As CrN thin films have very good oxidation, wear and corrosion resistant as compared to TiN, the Cr-B-N ternary coatings may be a promising candidate for tribological properties. As yet only very limited studies of Cr-B-N thin films have been carried out.

Newly developed rf inductively coupled plasma assisted planar magnetron sputtering apparatus is one of the most promising method for preparation of protective layers since it allows:(1)almost unlimited freedom to choose among elements, alloys and compounds as layering constituents;(2)to design and produce materials with compositional and structural periodicties approaching the atomic scale by sputtering several targets simultaneously;(3)to ensure minimal component distortion due to deposition temperatures being well below 500°C.paratraphNew Cr-B-N thin films were synthesized by rf inductively coupled plasma assisted planar magnetron sputtering apparatus in this study. The influence of nitrogen on the phase composition and mechanical properties of the Cr-B-N thin film is evaluated in this study. The XRD and XPS results indicate that h-BN phase appears in all Cr-B-N thin films and a phase transformation from CrB₂to BN and Cr₂N occurred in the Cr-B-N films with increasing the nitrogen partial pressure. Hardness and friction coefficient of the films were determined using nanoindentation and pin-on-disk apparatus, respectively. Both hardness and Young's modulus of the films decrease due to the formation of soft h-BN phase in the Cr-B-N films. However, h-BN shows itself as a kind of high efficient lubricant. The friction coefficient of the films decreases sharply from 0.8 to 0.2 when just a very small amount of nitrogen is introduced. These results indicate that it is possible to synthesize lubricating coatings with proper hardness and Young's modulus by controlling the coating composition to obtain the suitable proportion of the soft and lubricating h-BN phase.