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High Temperature Structural and Mechanical Properties of Hard Metal Doped CrNx and TiNx Coatings: Synchrotron Radiation and DFT Assisted Studies

Mohammadpour, Ehsan (2018) High Temperature Structural and Mechanical Properties of Hard Metal Doped CrNx and TiNx Coatings: Synchrotron Radiation and DFT Assisted Studies. PhD thesis, Murdoch University.

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This thesis presents a series of scientific studies exploring the thermal stability of CrN and TiN based metal nitride hard coatings. vacuum sputtered Cr1-xAlxN (14 <x<21 at.%), Cr1-xSixN (5 < x < 18 at.%), Cr1-xNixN (13<x<40 at.%) and Ti1-xSixN (0< x <4 at.%) thin film coatings, in the temperature range of 25 – 800 °C, were studied using (1) synchrotron X-ray powder diffraction beamline at Australian Synchrotron (SR-XRD), (2) nanoindentation tests, (3) X-ray photoelectron spectroscopy (XPS), and (4) Field Emission Scanning Electron Microscope (FESEM) imaging. Rietveld refinement method has been applied to SR-XRD patterns to quantitatively analyse the phase evolution and the changes of microstructure in the coatings. First principal calculations via density functional theory (DFT) and quasi-harmonic approximation (QHA) methods were used to calculate the structural and mechanical properties of metal nitrides at ground state and high temperatures up to 2800 °C. This study also presents a novel model which can predict the mechanical properties of such hard coatings at high temperatures.

Adding Al to CrAlN coatings improved microstructural and mechanical properties for ~14-21 % Al content. Rietveld analysis indicated various crystalline phases in the coatings that included: CrN, AlN, α-Cr with a small amount of AlO2 and Al2O3 over 25 – 700 °C range. Al doping improves resistance to crystal growth, stress release and oxidation resistance of the coatings. Al doping also enhances the coating hardness and elastic modulus up to 42 and 438 GPa. First-principles and QHA method on bulk CrN and AlN were incorporated to predict the thermo-elastic properties of Cr1-xAlxN thin film coating in the temperature range of 0 - 1500 °C. The simulated results at T= 1500 °C give a predicted hardness of H = ~41.5 GPa for a ~ 21% Al doped Cr1-xAlxN coating.

Adding Si to CrN reduced the crystallite size; stabilised the microstructure; modified coating softening resistance, and improves the hardness of CrN coatings up to 36 GPa. Although the coatings containing Ni have not shown a stable microstructure at high temperatures, changes in hardness and elastic modulus after heating are negligible. Formation of (Cr,Si)N and Ni(Cr) solid solutions are also postulated by studying the changes in lattice constants of CrN during heat treatment and comparing to estimated values from QHA calculations. The hardness of the CrN and Ni phases up to 930 °C was estimated by correlating the hardness with the bulk and shear modulus of these phases. The simulated hardness results predict hardness of H≈20 and ≈6 for CrN (930 °C) and Ni (330 °C), respectively.

The influence of substrate bias voltage, ranged from -30V to -80V, on the thermal stability and mechanical properties of the TiSiN coatings were investigated. As-deposited TiSiN coatings consist of cubic TiN in the form of (Ti,Si)N solid solutions. Increasing substrate bias voltage up to -80 V during Ti– Si–N deposition resulted in significant changes in Si content, surface morphology, phase composition, microstructure and Oxidation resistance of the coatings. TiO2 and Ti2O3 were identified above 600 °C for the samples deposited at lower bias voltages. Oxidation resistance and thermal stability of the TiSiN coatings were improved as the bias voltage increased beyond -40V. As the substrate bias voltage increases from -30V to -80V, Si content reduced from 4% to 0; hardness and Young's modulus of the coatings constantly improved from 23 GPa to 33 GPa and 310 GPa to 450 GPa, respectively, corresponding to almost 50% increase.

DFT-QHA algorithm was used to calculate phonon spectra, electronic properties, elastic constants (Cij) and thermodynamic properties of TiN within the temperature range of 0-2830 °C and under a pressure range 0-60 GPa. QHA method, as implemented in the Gibbs2 code, is utilised to accurately estimate thermal expansion coefficients, entropies, heat capacity values and Debye’s temperature. A similar approach was used to investigate electronic properties and stability phase diagrams the cubic boron nitride (c-BN) surfaces. Enthalpy of formation of c- BN was estimated to be -2.816 eV. The c-BN(100) surface offers separate B and N terminations, whereas c-BN(111) and c-BN(110) are truncated with combinations of boron and nitrogen atoms. Optimised geometries of surfaces display interlayer displacements up to the three topmost layers. Bulk c-BN and its most stable surface c-BN(100)_N possess no metallic character, with band gaps of 5.46 and 2.7 eV, respectively. We find that only c-BN(100)_B configuration exhibits a metallic character. c-BN(110)_BN and c-BN(111)_BN surfaces display corresponding band gaps of 2.5 and 3.9 eV and, hence, afford no metallic property.

The presented microstructural and mechanical analysis of metal nitride coatings using SR-XRD data along with the proposed methods to predict mechanical properties at high temperatures can be used to design superhard coatings with sophisticated microstructure and superior thermal stability at elevated temperatures.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Engineering and Information Technology
Supervisor(s): Jiang, Zhong-Tao, Dlugogorski, Bogdan and Altarawneh, Mohammednoor
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