Electron Microscopy Investigation of Detailed Microstructures of CVD TiAlN and TiN Coatings - Effects of Gas Flow and Substrate on Coating Microstructure

This thesis contains studies of topics of relevance to increase the understanding of TiN and TiAlN coatings synthesized by chemical vapour deposition (CVD) and low pressure CVD (LPCVD), respectively. This work contains two parts: (i) nanolamella LPCVD TiAlN coatings that were found to show a spatial variation of their chemical composition and microstructure, which was influenced by the gas flow speed of reactants; (ii) microstructure of CVD TiN coating deposited onto a CoCrFeNi multi-principal element alloy (MPEA)substrate.Depositing homogeneous TiAlN coatings on cutting tool inserts to achieve extraordinary mechanical performance and good chemical stability remains a challenging task. In this work, the detailed microstructure of high-Al cubic Ti1-xAlxN coatings (average x = 0.8) made by LPCVD was studied by state-of-the-art electron microscopy. A periodic structure with Ti-rich and Al-rich nanolamellae was found. Ti enrichment appears at high gas flow rate, which is suggested to be due to the increase of mass transport of certain reactants. However, the higher mass transport does not influence the Al-deposition much, as it is mainly limited by surface kinetics. A co-growth of h-AlN and TiAlN was investigated by electron microscopy, which showed that the h-AlN prefers to form on the Ti-rich cubic phase following a specific crystallographic orientation relationship: [1 1 0]cub // [2 -1 -1 0]hex, and (0 0 1)cub // (0 0 0 1)hex. This can be explained by a low interface energy between the h-AlN and Ti-rich cubic phase according to density functional theory (DFT) modelling. The microstructural inhomogeneity of TiAlN coatings can therefore be explained by the co-growth of the h-AlN phase with the Ti-rich cubic phase, which is caused by a fast gas flow. An electron microscopy study was done on the microstructure of TiN coatings grown on a CoCrFeNi MPEA substrate. The MPEA shows a good capability of serving as a CVD substrate. No severe etching of the substrate was found, and the growth of TiN was not disturbed by the substrate material. However, grain boundary diffusion of Cr into the TiN coating was revealed by analytical electron microscopy

Microstructural investigation of CVD TiAlN, TiN and WN coatings

High speed machining of workpiece materials puts extreme thermal and pressure loads onto the cutting tool inserts, which thus must possess both high hardness and good toughness. Nowadays, most cutting tools are made of cemented carbide substrates that are coated with wear-resistant coatings. Chemical vapour deposition (CVD) is a widely used industrial method for producing the wear-resistant coatings, and has advantages like conformal coverage of irregular shapes and high purity of the deposited materials. However, CVD is a complex process and the coating growth is therefore not fully understood.This thesis focuses on examining topics of relevance to increase the understanding of hard nitride coatings synthesized by CVD. The main research methods are analytical transmission and scanning electron microscopy (TEM and SEM), with complementary X-ray diffraction (XRD), atom probe tomography (APT) and simulations. Three types of CVD coatings were studied in this work: TiAlN coatings, TiN coatings and WN coatings.The work on TiAlN coatings: (i) First, the growth facets and texture were revealed. (ii) Second, an effect of precursor gas flow on the growth of the coating was studied, including the correlation between the formation of a nanolamella structure and the rotating precursor gas supply, and a microstructural inhomogeneity relevant to the varying gas environment. (iii) In addition, the full chemical composition of the TiAlN coating was studied via APT and electron microscopy. (iv) Finally, an intra-grain misorientation that forms in TiAlN during the CVD growth, and the formation of relevant dislocations, was studied. The work on TiN coatings: (i) The microstructure of the CVD TiN coatings deposited on a CoCrFeNi multi-principal elemental alloy (MPEA) substrate was studied, and (ii) the etching effect of the corrosive gas environment on the MPEA substrate was evaluated. The work on WN coatings: The microstructure and grain morphology of WN deposited on a (0001) sapphire substrate, especially the influence of deposition temperature on the microstructures, were studied.In conclusion, the results presented in this thesis provide insights into the detailed microstructures of TiAlN, TiN and WN coatings, which will increase the understanding of the growth mechanisms for these CVD coatings

The random lattice as a regularization scheme

A semi-analytic method to compute the first coefficients of the renormalization group functions on a random lattice is introduced. It is used to show that the two-dimensional $O(N)$ non-linear $\sigma$-model regularized on a random lattice has the correct continuum limit. A degree $\kappa$ of ``randomness'' in the lattice is introduced and an estimate of the ratio $\Lambda_{random}/\Lambda_{regular}$ for two rather opposite values of $\kappa$ in the $\sigma$-model is also given. This ratio turns out to depend on $\kappa$.Comment: PostScript file. 22 pages. Revised and enlarged versio

Memory effect and multifractality of cross-correlations in financial markets

An average instantaneous cross-correlation function is introduced to quantify the interaction of the financial market of a specific time. Based on the daily data of the American and Chinese stock markets, memory effect of the average instantaneous cross-correlations is investigated over different price return time intervals. Long-range time-correlations are revealed, and are found to persist up to a month-order magnitude of the price return time interval. Multifractal nature is investigated by a multifractal detrended fluctuation analysis.Comment: 11 pages, 4 figures