ICMCTF2019 Session B2-1-ThA: CVD Coatings and Technologies I

AlTiN coatings with high aluminum content have attracted attention due to its beneficial properties such as high oxidation resistance and age hardening effect on metal cutting. It is well-known that conventional AlTiN coatings are deposited by PVD method and the maximum aluminum content Al/(Al+Ti) is limited to lower than 0.67 to maintain a single phase cubic. Otherwise, in the range of aluminum content Al/(Al+Ti) higher than 0.67, they contain hexagonal phase which has lower hardness.

In this work, (111)- and (100)- oriented cubic AlTiN coatings with aluminum content higher than 0.67 were prepared on cemented carbide substrates by CVD method. Microstructure of the deposited coatings was observed by scanning electron microscopy, and the crystallographic orientation of these coatings was examined by X-ray diffraction analysis. Additionally, the hardness of these coatings was measured using nanoindentation and crack resistance property was evaluated by crack length induced by Vickers indentation, respectively.

From SEM observation, all of these coatings had columnar microstructures which contained self organised nano-lamellae structures in grains. Nano-lamellae structure in (100)-oriented coatings had a tendency to be smaller periods of the compositional fluctuation compared with those of (111)-oriented coatings, despite the almost same composition. As for the mechanical properties, the coatings with higher aluminum content or preferred (111) orientation showed higher hardness and higher crack resistance property. Thus, it was suggested that the hardness or crack resistance property of coatings was correlated to not only aluminum content but also the morphology of nano-lamellae structure.

One of possible explanation for the higher hardness, the deforming energy caused by indentation was dissipated by nano-lamellae structure. This effect is similar to a pinning effect for the propagation of the dislocations at interfaces of a multilayered coating. Regarding the superior crack resistance, it was attributed to crack deflection at the nano-lamellae structure.

Besides, these coatings were tested for the dry milling operation of 42CrMo4 steel blocks. It was indicated that CVD – AlTiN coatings with higher aluminum content showed superior wear and thermal crack resistance. On the other hand, the difference of coating orientation did not affect to the cutting results in this work.

To monitor the state of stress and damage in cutting tools or critical equipment working under extreme conditions such as high temperature, high pressure or aggressive chemical environment, the strongly textured AlN as a piezoelectric layer was successfully deposited on metals by Chemical Vapor Deposition (CVD) method. Low temperature textured TiN by CVD from TiCl₃-H₂-NH₃ system or AlN by Atomic Layer Deposition (ALD) is applied as a buffer layer to avoid the instability of metal substrates in CVD reactor during growth and decrease the huge mismatch between substrates and AlN layer. A design of experiment (DOE) is used to analyze the influence of 7 main process parameters and find a suitable experimental condition for depositing dense, covering and textured AlN. The morphology, XRD results and piezoelectric coefficient d₃₃ are compared and analyzed. Unexpectedly, the results show that (200) textured TiN is a good buffer layer to obtain (002) textured AlN on WC-Co. On the contrary, the AlN with the thickness of 90 nm deposited by ALD is the best buffer layer for TZM. At last, the relationship between texture coefficient and d₃₃ is studied considering 10 different orientations of AlN. It proves that when (002) texture coefficient is less than 2, d₃₃ almost keeps constant at 0.14 pC/N, then it increases linearly to more than 2 pC/N while the theoretical bulk piezoelectric coefficient for AlN is around 6 pC/N.

There is a growing interest in concentrating solar power plants as electricity generation systems, whereby the sunlight is redirected and focused onto a receiver heated to high temperature. One of the challenges is to build the solar receiver which can work at temperatures near or higher than 1000 °C for optimizing the yield. Current candidate materials are metallic alloys such as Inconel, or bulk ceramics like silicon carbide, but their operating temperatures may be limited due to oxidation or mechanical problems. Aluminum nitride (AlN) coating, deposited by chemical vapor deposition at 1100 °C, was selected for its high thermal conductivity, low thermal expansion coefficient, high temperature stability and its oxidation resistance. It forms stable and protective alumina scales at temperatures higher than 1000°C. Oxide dispersion strengthened (ODS) FeCrAl alloy (Kanthal APMT), also alumina-forming, was chosen as a model substrate to study the potential of AlN coatings. Accelerated cyclic oxidation and high temperature emissivity measurements were performed in Odeillo solar furnace facilities (France), confirming the potential of aluminum nitride coatings as materials for high temperature central receivers. The AlN based multilayered system exhibits low degradation after 1500 h of oxidation at 1100 °C in air. The modelling and simulation of stresses during thermal cycles taking into account the creep and growth of the oxide layer are used to show the limits of use of these materials.

Transition metal dichalcogenides (TMD) like WS₂, WSe₂, MoS₂ and MoSe₂, when thinned to a monolayer (ML) thickness show direct bandgap, valley spin polarization, reasonable charge carrier mobilities and relatively high optical absorption; properties relevant to many applications. In addition to binary materials, alloys of TMD materials are also promising. It was reported that electronic transport properties are not dramatically deteriorated for the TMD alloys, and alloying allows for bandgap engineering over a wide range between 1.56 eV (794.8 nm) for MoSe₂ and 2.01 eV (590.4 nm) for WS₂. To push these materials towards practical applications, wafer-scale epitaxial growth of ML and few-layer (FL) films is necessary which can be achieved using CVD. In this study, to achieve ML and FL epitaxial layers of TMD materials and their alloys, we use gas source CVD and sapphire ((0001) α-Al₂O₃) as substrates.

For the growth we employ metal hexacarbonyls (W(CO)₆ and Mo(CO)₆) and hydrides (H₂S, H₂Se) as precursors diluted in hydrogen (H₂) carrier gas. Deposition temperature for this class of materials is in the range between 800 and 1000 °C at a pressure of 50 Torr for sulfides and 200 Torr for selenides. For the growth of binary compounds, a previously developed multistep growth process was employed while for the alloys, a single step was implemented to minimize routes for the formation of compositional gradients in the resulting layers.

As a result, we obtained ML and FL films of TMD materials. Acquired samples showed epitaxial relation with the substrate, high intensity photoluminescence and, for the alloys, composition control was achieved over a wide range as determined using X-ray photoelectron spectroscopy. Growth of WS₂ films resulted in growth of epitaxial ML films ((10-10)WS₂//(10-10)α-Al₂O₃) with in plane twist of 0.1°, room temperature photoluminescence (PL) showed emission maximum at 2 eV and width of 0.04 eV with pronounced exciton and trion components as well as defect-bound exciton emission. Domain size and distribution of antiphases were studied using transmission electron microscopy (TEM). This showed a relatively low fraction of isolated antiphase regions.

WMoS₂ alloys showed variation in the PL peak position as well as a change in the Raman scattering spectra which can be correlated with the composition of the film. Microstructure of the films was studied using TEM that showed no distinct segregation of the metal atoms when continuous flows of metal precursors were used.

Keywords: CVD, Ti(B,C)N, hard coatings, TEM, micromechanical testing