SYNTHESIS AND DEVELOPMENT OF HARD AND TOUGH NANOCRYSTALLINE METAL NITRIDE COATINGS

Abstract

Nanocrystalline metal nitrides coatings have been successfully applied on various industrial components since the 1970’s, for the protection and particularly to enhance the life of components. The metal nitrides coatings studied for tribological application are mainly based on transition metal nitrides includes TiN, CrN, TaN, ZrN, W2N and their alloys like TiAlN, TiCrN, HfON, ZrON, TiAlBN, WSiN, nc-TiN/a-Si3N4 and so on. Most of them are commonly referred to as refractory hard metals. But, the hardness alone may not provide the level of protection against wear damage. In addition to high hardness, high toughness of a coating also has equal importance especially for tribological applications where high normal and shear forces are present. A coating with high toughness has high resistance to propagation and formation of cracks under stress. Moreover, high energy absorbance capability of tough coatings prevents catastrophic failure. Hard nanocrystalline coatings with enhanced toughness exhibit a high elasticity (resilience), an enhanced resistance to cracking and low plastic deformation. Multicomponent refractory material systems can provide opportunities for the synthesis of hard and tough coatings. The development nanocrystalline metal nitride coatings by various physical deposition techniques is ever growing to achieve their superior performance in the actual engineering applications. The literature on the optimized process variables using magnetron sputtering technique for the deposition of hard and tough metal nitride coating with the desirable microstructural characteristics is very limited. The control of the microstructural characteristics in terms of grain size, lattice defects, crystallographic orientation (texture) and surface morphology as well as phase compositions of these coatings are very important in realizing the aforementioned properties so as to extend the performance and life time of the coated products. The main objectives of present research work are to synthesize hard and tough nanocrystalline transition metal nitride coatings such as Zirconium Tungsten Nitride (Zr-WN), and Zirconium Tungsten Boron Nitride (Zr-W-B-N) coatings using DC/RF magnetron sputtering technique and to investigate the effect of sputtering process parameters on microstructure, thermal stability and mechanical properties of these coatings for obtaining high quality films. A chapter-vise summary of the thesis is given below: II Chapter 1 The potential application of present research work is centered on prevention of wear damages like cavitation and silt erosion in hydro turbine components using hard and tough nanocrystalline metal nitride coatings. Hence this chapter discusses about hydropower generation in India, erosion problems facing in electricity generation by hydro turbines and approaches used to encounter silting problems. This chapter also gives an overview of metal nitride coatings in which background, synthesis techniques and applications are discussed. A role of hardness and toughness to control wear damages has been discussed briefly. The structural, thermal stability and mechanical properties of zirconium nitride, tungsten nitride, zirconium tungsten nitride and zirconium tungsten boron nitride coatings and their applications are discussed in this chapter. The motivations, research gaps and objectives for the development of hard and tough nanocrystalline zirconium tungsten nitride and zirconium tungsten boron nitride coatings are also discuss in Chapter 1. Chapter 2 deals with the synthesis and characterization of “thin films” and describes the process of DC/RF magnetron sputtering used for the fabrication of thin films in the present work. Process and mechanism of thin films growth have been explained to understand how a film actually grows on a substrate. Three different modes of thin film growth (layer, island and layer + island) have been explained in detail. A description of working principles of different characterization techniques such as X-Ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM), Wavelength dispersive spectroscopy (EDS), Energy dispersive spectroscopy (EDS), Nanoindentation and Microindentation used for the analyses of elemental composition, phase, textures, grain size, surface morphology and mechanical performance of the deposited films have been given in detail. Chapter 3 describes the effect of nitrogen partial pressure (pN2) (0.07 to 0.67 Pa) on structural, composition and mechanical properties of ZrxW1-xNy thin films. It has been observed that the structure and elemental composition of the deposited ZrxW1-xNy thin films strongly depend on pN2. XRD analysis shows that for 0.07 Pa ≤ pN2 ≤ 0.17 Pa, ZrxW1-xNy films exhibit single (fcc) phase, for 0.20 Pa ≤ pN2 ≤ 0.27 Pa, an amorphous phase is obtained and for 0.33 Pa ≤ pN2 ≤ 0.67 Pa reflections corresponding to fcc phase Zr-W-N and hcp phase ZrN have been observed. The phase formation has been confirmed by TEM diffraction patterns (SAED). The diffuse rings in SAED pattern of film deposited at pN2 = 0.27 Pa confirm that nanocrystalline grains were not grown during deposition. The film deposited at pN2 = 0.47 Pa exhibit maximum III (6.8 nm) roughness because the XRD peaks of ZrN are relatively prong with the XRD peaks of Zr-W-N phase which indicate that ZrN phase is well established along with Zr-W-N phase and hence roughness increases. Results of nano-indentation analysis confirm moderate hardness, high wear resistance, high resistance to plastic deformation and high adhesiveness of ZrxW1-xNy films. Among all the phases, maximum hardness (~ 24 GPa) and maximum reduced elastic modulus (135 GPa) have been obtained for dual phase (fcc+hcp) film while resistance to fatigue fracture (H3/Er 2 ~ 0.87 GPa), wear resistance (H/Er ~ 0.2) and ductility for single phase (fcc) film were found to be maximum. All the films were found to exhibit high adhesion with the substrate surface. Chapter 4 discusses about the structure, thermal stability and mechanical properties of crystalline (fcc) phase and amorphous phase Zr-W-N thin films. This chapter divided into two sections. Section 4.1 describes the effect of substrate temperatures Ts (100º-600ºC) and post annealing temperature Tn (100º-600ºC) on thermal stability and mechanical properties of amorphous phase Zr19W18N63 thin films. For 100ºC ≤ Ts≤ 300ºC, XRD patterns show an amorphous structure of the films and for 400ºC ≤ Ts≤ 600ºC, a crystalline fcc phase with (111) and (200) orientation has been observed. The XRD findings are further confirmed by TEM and SAED patterns. Maximum wear resistance (H/Er ~ 0.22) and maximum resistance to fatigue fracture (H3/Er 2 ~ 1.1 GPa) have been obtained for the amorphous films deposited at Ts = 200ºC. Post annealing of films deposited at 200ºC have been carried out in air from 100º- 600ºC. Oxygen starts to be incorporated in the films at Tn = 300ºC and its content increases with increasing Tn. No crystalline oxide phases are observed up to Tn = 600ºC. The hardness of the annealed films decreases with increasing oxygen incorporation. Indentation and scratch tests for as deposited and annealed films show that no cracks propagate in the films even at a high load of 50 mN and all the films exhibit high adhesion with the substrate. Section 4.1 describes the effect of substrate temperatures Ts (100º-600ºC) and post annealing temperature Tn (100º-600ºC) on thermal stability and mechanical properties of crystalline fcc phase Zr22W19N58 thin films. For 100ºC ≤ Ts ≤ 600ºC, X-ray diffraction patterns of the deposited films show a crystalline fcc phase with (111) and (200) preferred crystallographic orientations of grains. A close analysis of the diffraction pattern shows that for low Ts (< 400ºC) the coating structure is not simply fcc but probably contains some amorphous parts along with the fcc phase. It can be clearly seen that the contribution of amorphous part is maximum for Ts =100ºC, decreases with increasing Ts and disappears for Ts ≥ 400ºC. The XRD findings are further IV confirmed by TEM and SAED patterns. Maximum wear resistance (H/Er ~ 0.22) and maximum resistance to plastic deformation (H3/Er 2 ~1.0 GPa) have been obtained for the film deposited at Ts = 400ºC. Post annealing of the films deposited at 400ºC have been carried out in open atmosphere at 100º-600ºC. For the annealed films, no crystalline oxide phase has been detected for 100ºC ≤ Tn ≤ 600ºC, even though oxygen incorporation in the films starts at Tn ≥ 300ºC. The films start peeling off at 500ºC and got completely peeled off at 600ºC. The crystallite size increases with increasing Tn and reaches a maximum value of ~10 nm at Tn = 400ºC. Hardness and elastic modulus of annealed films found to be increasing with increasing strain. For practical applicability, the film deposited at 400ºC is found to be most suitable for application in the temperature range below 300ºC. But the deposition of wear protective coatings at low substrate temperature (≤ 200ºC) is more feasible for commercial aspects. Mechanical properties of amorphous phase Zr-W-N coatings are found superior to crystalline phase Zr-W-N coatings. But Zr-W-N coatings exhibit amorphous phase at low Ts ≤ 300ºC which restricts the further enhancement of physical and mechanical properties of Zr-WN coatings either by varying other sputtering parameter likes negative biasing (chapter 5) or by changing the architecture of coatings through substitution of amorphous non-metal nitride phases (chapter 6). Hence, crystalline (fcc) phase Zr-W-N coatings has been selected for further study in the present research work. In Chapter 5 effect of negative substrate bias voltage Vs (-20 V to -120 V) on structure and mechanical properties of fcc phase Zr-W-N coatings deposited at 200ºC substrate temperature has been studied in details. The application of negative bias voltage to the substrate leads to impingement of energetic ions on the coating surface is an effectual way to grow dense and void-free microstructure of coating at low deposition temperatures. The deposition rate and substrate ion current density vary non-monotonically with increasing Vs. XRD patterns of the Zr-W-N coatings revealing one group of peaks of fcc structure, indicating that this coating tends to form a single solid-solution nitride phase rather than the co-existence of separated nitrides. FESEM analysis shows that the morphology evolves from columnar structure to dense and then glassy structure with increasing negative bias voltages. Nanoindentation hardness H and effective elastic modulus Er of the coating increases as the negative substrate bias goes up. Maximum wear resistance (H/Er ~0.23) and fracture toughness (KIC ~ 2.25 MPa.m1/2) have been obtained for the film deposited at -100 V bias voltage. This indicates that the simultaneous increment of wear resistance and toughness is achievable if the negative bias voltage is V properly controlled. Therefore, for the deposition of Zr-W-N system done at low substrate temperature (200ºC) and at -100V bias voltage, pronounced mechanical properties (H~ 34 GPa, H/Er > 0.1, We > 60%, H3/Er 2 > 0.1, KIC ~ 2.25 MPa.m1/2) has been achieved. Chapter 6 discusses about the effect of microstructure on thermal stability and mechanical properties of Zr-W-B-N nanocomposite coatings. The Zr-W-B-N nanocomposite coatings have been co-sputtered deposited on Si (100) substrates by varying power density (0.1 to 7.5 watt/cm2) to boron target to obtain films of various compositions and microstructure. It has been observed that the Zr-W-B-N films with boron contents ≤ 2.3 at.% exhibited (200) preferred crystallographic orientation of grains and columnar structure. For the boron content ≥ 7.5 at.%, non-columnar films with the crystal phase grain size less than 7 nm and films of crystalline (Zr-W-B-N)-amorphous structure (BN) or high amorphous component are produced. Owing to synergetic contribution of solid solution strengthening and grain boundary hardening, film with boron content ~7.5 at.% exhibits maximum hardness (~ 37 GPa), wear resistance (H/Er ~ 0.24) and fracture toughness (2.9 MPa.m1/2). Due to the superior mechanical performance of Zr-W-B(7.5 at.%)-N film over other deposited films of varying B content, post annealing of the this films has been carried out at 300º-900ºC in vacuum (Tv) and in air (Tn). Zr-W-B(7.5 at.%)-N film retains the fcc structure after vacuum annealing at Tv = 900ºC. However, film retains the fcc structure during air annealing at Tn ≤ 700ºC and at Tn = 900ºC, full degradation of the Zr-W-B(7.5 at.%)-N film in crystalline phases of ZrO2 and WO3 was observed. The oxygen starts to be incorporated in the film at Tn = 500ºC and its content increases substantially with increasing Tn. The film got completely peeled off at Tn = 900ºC. Hardness H and elastic modulus Er of the films remains unaffected by vacuum annealing while increased with increasing oxygen concentration in to the films. Chapter 7 summarizes the results and findings of the present work discussed in this thesis. The future directions in which these studies can be extended have been suggested at the end.