ICMCTF2012 Session B2-1: B2-1-:CVD Coatings and Technologies

The ever increasing demand for higher productivity in manufacturing requires advanced hard coatings. The coatings can be tailored using PVD and CVD processes to exhibit for example higher hardness and / or enhanced oxidation resistance and good adhesion. Nowadays most PVD coatings based on the Ti-Al-N system with addition of silicon, chromium, boron or yttrium and are commercial available. The system Ti-Al-N offers a very good combination of a hardness and ductility. PVD coatings offering compared to CVD coatings compressive stresses without any post treatment and high deposition rates. But the development horizon on the PVD Ti-Al-N system is limited, because starting at around 66at% Aluminium content a phase change from the cubic to the wurzite structure can be seen.

On the other side CVD processes offering a good adhesion and are very stable process behaviour especially at the deposition of oxide coatings. Also they offering compared to the PVD processes even using the new HIPIMS process a nearly homogenous coating distribution around the cutting edge. Depending on the insert geometry a high coating thickness on the cutting face offering high wear resistance. Since the last couple of years no new hard coatings systems were investigated and developed in the CVD sector. The development focus was often - working on the stress behaviour in the coating or the coating architecture.

Today in the field of cast iron cutting PVD-TiAlSiN or even PVD-AlTiN coatings with a high hot hardness compete against well establish CVD TiN/TiCN/alpha-Al₂O₃ or kappa-Al₂O₃ coatings gaining more and more market share in the milling and also in the turning operation.

The logic development way is now the combination of the good things from both worlds: the system Ti-Al-N from PVD and the good adhesion and coating distribution from CVD.

By using a new developed the CVD process it is possible to deposit cubic AlTiN-CVD coatings even above 70% Aluminium with superior coating properties.

In this regard this coating family was analysed using common thin film techniques revealing hardness, Young´s modulus and coating adhesion. This new develop coating is tested in milling experiments in cast iron comparing conventional state of the art hard-coatings. The developed coating shows a significant increase of tool life even at higher cutting speeds.

The microstructures of four different CVD Deposited α‐Al₂O₃ and TiC_xN_(1‐x) coatings were determined by cross sectional electron backscatter diffraction mapping. The harnesses of the layers were also measured by nanoindentation. Using the microstructural data as input, two‐dimensional finite element analysis was used to calculate the residual thermal stresses in these materials. The thermal stresses and stored elastic energy in the α‐Al₂O₃ layer are larger than those in the TiC_xN_(1‐x) layer. Furthermore, the mean value and distribution of stored elastic energy are influenced by the texture in the alumina layer. Coatings with weaker texture have a broader distribution of thermal stresses. Coatings with alumina oriented so that the [0001] direction is parallel to the film growth direction have less stored elastic energy. This is because the thermal expansion perpendicular to [0001] is less than the thermal expansion parallel to [0001] and, therefore, the thermal expansion mismatch between the alumina coating and the substrate is minimized when grains are oriented with [0001] perpendicular to the substrate. The thermal stresses in hypothetical coatings with synthetic microstructures were also computed. These calculations tested the effects of coating thickness, channel crack spacing, composition of the TiC_xN_(1‐x) layer, grain aspect ratio, and cobalt enrichment of the substrate on the thermal stresses. Based on the thermal stresses, it is concluded that the three most significant factors influencing coating hardness, ordered from most significant to least significant, are the composition of the TiC_xN_(1‐x) layer, the channel crack spacing, and the cobalt enrichment of the substrate.

Al₂O₃ and TiCN coatings have been widely used in cutting tools. TiCN has high hardness. Al₂O₃ maintains high hardness and excellent oxidation resistance under such a severe cutting condition. Moreover, it is well known that the orientations of these coatings effect their performance. For instance, specific oriented Al₂O₃ represents higher performance than the other orientations. These coatings are deformed during cutting, and wear out at last. Therefore it is necessary to investigate the deformation mechanism of such ceramics coatings.

In this study, (422) and (220) oriented TiCN coatings and (006) oriented Al₂O₃ coatings with low and high CSL boundaries are prepared. These samples are indented with Micro Vickers . Deformed areas are observed in 3 dimension s with FIB and EBSD system. The effects of orientations and grain boundaries on deformation are discussed in details.

TiN and TiC_xN_y are commercial CVD coatings widely used for cutting tool applications. A promising route for improving hardness and oxidation resistance is the addition of silicon. Thermal CVD method was employed using gas mixtures containing TiCl₄ and the silicon chlorides SiCl₄ or Si₂Cl₆. This work is focussed on the investigation of structure, composition and properties of the TiSiN and TiSiCN coatings deposited on hardmetal inserts.

TiSiN layers with a nanocomposite structure were obtained with SiCl₄ as well as Si₂Cl₆ in a temperature range between 800°C and 900°C. In both cases ammonia was used as nitrogen precursor. The crystalline phases TiN, and at 900°C Ti₅Si₃ as well, were analysed by XRD. From the TEM investigation of a layer deposited at 850°C it is evident that the nanocrystalline TiN is embedded in an amorphous phase. The amorphous phase is silicon nitride. The hardness correlates well with the silicon content and the grain size. A maximum hardness about 37 GPa was observed at a silicon content between 6 and 8 at.% if SiCl₄ was applied as silicon precursor. In this silicon concentration range a TiN grain size of 14.5 nm was determined. If Si₂Cl₆ was used the hardness maximum of 38 GPa was already achieved at a lower silicon content of 3.5 at.%. The corresponding TiN grain size is 16.4 nm. The investigation of the oxidation behavior showed an increase of the oxidation resistance with the silicon content. In the case of layers with silicon contents between 5 and 8 at.% an oxidation resistance up to 700°C was observed. At this temperature a weak TiO₂ formation occurred at the surface.

Furthermore TiSiCN coatings were deposited using the same titanium and silicon precursors. The ratios of the silicon precursors to TiCl₄ were varied. Hardness values up to 40 GPa were measured for optimum ratios. The structure was analysed by SEM, XRD and XPS.

The examination of the adherence of TiSiN and TiSiCN layers showed that a diffusion barrier is necessary for suppressing the cobalt diffusion from cemented carbide substrate into the layers. If interlayers of TiN or TiCN were applied critical loads of 80 N for TiSiN and 50 N for TiSiCN layers were obtained from scratch test measurements.

Highly crystallized silicon layers were grown on metal sheets at high temperature (950°C) by thermal CVD from silane. An intermediate buffer layer (TiN layer) was mandatory to prevent interdiffusion and silicide formation but also to compensate lattice parameters and thermal expansion coefficients mismatches between metal and silicon and ideally transfer some crystalline properties (grain size, texture) from the substrate to the silicon layer. After a thermodynamic study, intermediate titanium nitride diffusion barrier was selected and processed by CVD. Special attention is given to the substrate surface preparation for texture transfer. The structure and the interfaces stabilities of these silicon/nitride/metal stacks were studied by FEG and TEM, X-ray diffraction, Raman and energy dispersive X-ray spectroscopy.

By both optimizing substrate preparation and silicon processing conditions, this multilayered structure could be able to provide an efficient and reliable converter, comparable with classical crystalline silicon wafers in terms of solar conversion yield, while overcoming their majors drawbacks: due to ingot sawing and squaring as well as wafer slicing.

In this work, we present the plasma copolymerization of Maleic Anhydride (MA) and Vinyltrimethoxysilane (VTMOS) performed in an Atmospheric Pressure-Dielectric Barrier Discharge process for the formation of highly reactive anhydride functionalized coatings.

In the first part of the presentation, we will show how the tuning of the electrical parameter led to coatings with different combination of anhydride/carboxylic group surface density, morphology and deposition rate. For that goal, the power (P) delivered by the source was varied from 50 to 150W. Moreover, the discharge electrical mode was operating in a continuous wave or a pulsed wave with a pulse ON-time (t_on) fixed at 10ms and a pulse OFF-time (t_off) ranging from 10 to 60ms. The different coating chemistries have been studied by XPS for the estimation of the anhydride/carboxylic surface density and by ATR FT-IR to investigate the conservation or the destruction of the cyclic anhydride group related to the incorporation of the MA monomer in the coatings. The average power (P_av), defined as P_av=P.[t_on/(t_on+t_off)], appeared as a key parameter to control the anhydride surface functionalization within a range of 2 to 13 at.%, independently of the coatings morphology or deposition rate.

In the second part, we will present how the coatings reactivity has been exploited for the elaboration of a thermally reversible interfacial bonding property based on a Diels-Alder reaction. For that, MA-VTMOS deposits have been performed on a rigid polished aluminium and on a kapton foil and subsequently, treated through a chemical gas phase reaction for the grafting of the required diene and dienophile groups. The complete chemical path reaction will be exposed with some adhesion results dealing with the Diels-Alder and the retro-Diels-Alder reactions.

Zinc oxide (ZnO) is useful for many applications such as transparent conductive films in solar cells and flat panel displays, optoelectronic devices operating at short wavelengths. Despite the advantages of MOCVD for industrial applications, ZnO film growth by conventional MOCVD consumes a lot of electric power to react the source gases and raise the substrate temperature. To overcome this, a more efficient means of reacting oxygen and metalorganic source gases is needed. In addition to the low reaction efficiency, conventional CVD methods yield low-quality ZnO films, due to incomplete reaction of metalorganic source gases with oxygen source gases in the gas phase. If thermally excited water is used to hydrolyze the metalorganic source gases, however, reactive ZnO precursors are produced in the gas phase, allowing growth of ZnO films under energy-saving conditions.

In this study, we present a ZnO film growth on glass substrates aiming at the application for transparent conductive thin films, using the reaction between dimethylzinc (DMZn) and high-energy H₂O produced by a Pt-catalyzed H₂-O₂ reaction. CVD apparatus is comprised of a catalytic cell with a fine nozzle, gas lines for H₂, O₂ and DMZn supplies, and a substrate holder. H₂ and O₂ gases were admitted into a catalyst cell containing a Pt-dispersed ZrO₂ catalyst, whose temperature increased rapidly to over 1300 K due to the exothermic reaction of H₂ and O₂ on the catalyst. The resulting high-energy H₂O molecules were ejected from the fine nozzle into the reaction zone and allowed to collide with DMZn ejected from another fine nozzle. ZnO films were grown on glass substrates at 573-873K and the film properties were evaluated.

X-ray diffraction patterns of the ZnO films grown at 673-873K exhibited intense (0002) peak at 2q=34.42-34.47˚, which means that small tensile stress exists in the ZnO/glass films grown in our method. Average transmittance in the wavelength between 500 nm and 2000 nm was higher than 85%.

In the photoluminescence (PL) spectra measurement at room temperature, band edge emission of ZnO films appeared at 3.27-3.30eV. The PL spectra at 17.8K showed the strongest emission peaks at 3.357-3.360eV, which are attributed to the neutral-bound excitons D^o_x. The FWHM value of the strongest peak of the film grown at 673K was as low as 6.34 meV, which is smaller than that previously reported for ZnO (10meV) obtained by MOCVD grown on sapphire substrates (using Zn(C₂H₅)₂ and N₂O as source gases).

Three nano-architectures of indium oxide (In₂O₃) have been successuflly synthesized by thermal evaporation, which include nanorods, nanotowers, and tin-doped indium oxide (ITO) nanorods. Judging from the images of transmission electron microscope (TEM) analysis and the x-ray diffraction (XRD) analysis, the slight dopping of tin did not affect and change the growth direction and in-situ micro-structure of nano indium oxide. The qualitative analysis of ITO nanorods were analysized by energy dispersive spectrometer (EDS) and x-ray photoelectron spectroscopy (XPS), which confrimed that slight tin were totally dpoed into nanorods of indium oxide via thermal evaporation. The optoelectronic properties of In₂O₃ and ITO nanorods were tested by cathodoluminescence (CL) analysis. There are blue shifts in the CL spectrums, which also means tin was definitely added in In₂O₃ nanorods. The results of I-V curve show that the resistance of nanorods, nanotowers and ITO are 1.32 k Ω, 0.65 k Ω and 0.063 k Ω . According to this result, dopping slight tin is a good approaching to enhance the conductivity of In₂O₃ nanorods.