First-principles Calculations On The Thermodynamic And Mechanical Properties Of Ti-Al-(Zr, Hf)-N Wear-resistant Coating Systems

Al-based nitride coatings are widely used as the cutting and forming tools due to their high hardness and wear resistance, high corrosion, as well as good thermal stability. It is reported that the miscibility gaps exist in the Al-based nitride Ti-Al-N, Ti-Zr-N, Zr-Al-N and Hf-Al-N systems. As a result, these ternary systems exhibit spinodal decompositions at elevated temperatures, and the thus cubic binary nitrides TiN, AlN, ZrN, and HfN can be formed through thermal decomposition, resulting in age-hardening of coatings. This kind of age-hardening effect can improve the mechanical properties of coatings. Therefore, the investigations of spinodal decompositions as well as the properties of the formed cubic binary nitrides play an important role in the design of high performance coatings. Additionally, in order to improve the properties of coatings, the development of quaternary coatings through introducing new alloy elements X's (X=Zr, Hf, Cr, Nb, Ta, Si, Zr, and Y etc) into Ti-Al-N is a current research focus. Unfortunately, a huge amount of work is needed to select the right alloy elements by using experimental method, while first-principles calculations on the investigation of structural and mechanical properties can reduce the workload effectively and provide the reasonable explanation for the experimental observation.Based on the first-principles calculation coupled with quasi harmonic approach and efficient strain versus stress method, the structural, thermodynamic and mechanical properties of binary cubic nitrides systems have been systematically investigated. The elastic properties as well as the effects of pressure and vibration on the spinodal decomposition of cubic TiAIN, TiZrN and ZrAIN alloys have been computed using first-principles calculations. Herein, the cluster expansion (CE) method and especially the special quasirandom structures (SQS) are employed to describe the ternary disordered alloys. Additionally, the quaternary SQS model has been developed in the present work, and the developed model is employed to predict the structural and mechanical properties of quaternary TiAl(Zr, Hf)N. The qualitative analysis for the effect of the addition of Zr and Hf on the mechanical and spinodal decomposition of TiAIN coating has been also performed in the present work. The present thesis consists of the following four parts:(1) The mechanical properties of ternary nitride coatings can be improved by the spinodal decomposition into cubic binary nitrides. Therefore, the investigation of thermodynamic and mechanical properties of cubic binary nitrides is very important. In the present work, structural, phonon, electronic, and thermodynamic properties of cubic binary nitrides MN (M=Ti, Al, Zr and Hf) have been systematically investigated by first-principles calculations. The present thesis provides an accurate prediction of thermodynamic properties at high temperatures, especially for the thermodynamic data which are difficult to be measured. The present results agree well with the available experimental data, and can provide necessary thermochemical data for CALPHAD (CALcalculation of PHAse Diagram) modeling.(2) Residual stress is one of the important factors, which affect the mechanical properties of coatings. However, it is difficult to directly measure the residual stress via experiments. In the determination of stress, one usually measures the lattice distortion by x-ray diffraction first, and then calculates the stress using elastic constants. Therefore, the elastic stiffness constant plays an important role in determining the residual stress. Based on the first-principles quasiharmonic approach and an efficient stress-strain method, temperature-dependent and pressure-dependent elastic constants are predicted for nitrides for the first time, which is more convenient to determine the residual stress of coatings. Additionally, based on the temperature-dependent elastic constants, the present work also predicts other mechanical properties, such as bulk modulus, shear modulus, Young's modulus, fracture strength, and hardness, of cubic nitrides MN (M=Ti, Al, Zr and Hf). These results can serve as reference data for the development of coatings at high temperatures.(3) The age-hardening caused by spinodal decomposition improves coating performance. Accurate predictions of spinodal decomposition curves play an important role in the study of coating age-hardening. Here, the SQS and cluster expansion (CE) methods are used for the treatment of ternary disorder phase. The effects of both lattice vibration and pressure on the thermal decompositions of TiAlN, ZrAlN, and TiZrN have been studied for the first time. It is found that the temperatures of the predicted binodal and spinodal curves increase with increasing pressure, while the vibration contribution decreases significantly these temperatures. In addition, the present study indicates that the composition range of the binodal and spinodal curves of cubic Ti-Al-N coatings can be enlarged by adding Zr.(4) The available SQS are limited to the description of binary and ternary disordered phase. The quaternary special quasirandom structures (SQS's) have been developed in the present work to describe the disordered solid solutions, and in turn, the mechanical properties of Ti-Al-Zr-N and Ti-Al-Hf-N coatings are calculated. Additionally, the effect of alloying elements Zr and Hf on the spinodal decomposition of Ti-Al-N is studied. It is found that the addition of Zr and Hf can change the starting direction of the spinodal decomposition of Ti-Al-N from <100> to<111>. By analyzing the electronic density of states and the enthalpies of mixing for ternary nitrides, it is concluded that Zr and Hf are effective to increase the age-hardening of Ti-Al-N coatings due to the enlarged composition range of spinodal decomposition. The first-principles calculation is proved to be as an effective and reasonable approach for selecting elements, optimizing alloy component and predicting alloy properties.