Super-hard Tough Coatings by Crystal Defect Engineering

Abstract

Coatings technology has the potential to improve the wear and corrosion resistance of materials, leading to increased service life of engineering components. Higher coating hardness is typically desired due to its ability to provide superior protection. One common approach to enhancing coating hardness is through grain refinement according to the Hall-Petch effect. However, an increase in hardness often comes at the cost of reduced toughness, which can result in sudden failure under large external impact. While various toughening strategies, such as ductile phase toughening, fiber toughening, multilayer toughening, and phase transferring toughening, have been proposed, none have fully resolved the trade-off between hardness and toughness. In recent years, there has been growing interest in tailoring material properties at the crystal lattice level. This approach has been shown to produce unique crystal defects, such as special grain boundaries, stacking faults, and dislocations, which can lead to unprecedented properties, including increased hardness and toughness simultaneously. However, generating these crystal defects typically requires post-mechanical processing or rapid cooling, which is difficult to apply to ceramic coatings. This thesis aims to explore different strategies for the controllable production of crystal defects and investigate their effects on the properties of coatings, particularly hardness and toughness.

To produce high-density crystal defects, plasma immersion ion implantation and deposition (PIII&D) is employed. PIII&D is shown to introduce high-density point defects and stacking faults into the CrN coatings, particularly at a bias exceeding 10 kV. The dislocation density significantly increases from 5.2×10¹⁴/m² to 1.3×10¹⁷/m², and the covered area of defects increases from 30% to 75% with ion implantation. The hardness of the coatings significantly increases from 24.2 GPa to 35.6 GPa, but the fracture toughness does not improve as intended due to the appearance of clusters of point defects.

In the next chapter, post-annealing is performed on the ion-implanted CrN samples to eliminate the clusters of point defects. The crystal structure evolution and properties are systematically investigated. It is found that the coatings are composed of stacking faults and clusters of point defects that tend towards amorphization. The existing stacking faults gradually grow at low temperatures below 350℃ and disappear at higher annealing temperatures. The clusters of point defects transfer to stacking faults after annealing at 350℃ and also change into a normal crystal at 500℃. The proportion of stacking faults reaches a maximum at 350℃, attributed to the recovery of supersaturated interstitial atoms during annealing. As a result, the hardness of the coatings reaches a super-hard status of 40.7 GPa, and the toughness simultaneously enhances to 1.56 MPa·m^1/2 after annealing at 350℃.

In the following part, the thesis explores the methods in preparing coherent grain boundaries to further improve the toughness. Periodic production of hetero-interfaces in CrN coatings through energetic ion bombardment is proposed. These hetero-interfaces alter crystal growth kinetics and enable a preferred orientation transition from (111) to (200) orientations near the interfaces, resulting in the formation of coherent grain boundaries (CGBs). By optimizing sub-layer thickness, the quantity and width of CGBs increase significantly, leading to a significant improvement in the hardness and toughness of the CrN coating, which reach values of 33.4 GPa and 3.23 MPa·m^1/2, respectively. The effect of different CGBs on coating strength is also discussed.

Based on the previous results, an amorphous surrounded nano-crystallized TiC coating is designed, which exhibits high compressive stress. Tiny stacking faults are observed in the as-deposited sample, and after annealing, these stacking faults begin to grow under the influence of compressive stress. Additionally, the coalescence and growth of nanocrystals during annealing promote the formation of coherent grain boundaries between different grains. The formation of stacking faults and coherent grain boundaries significantly increases the hardness from 28.3 to 41.3 GPa. The fracture toughness, as tested by indentation methods, also displays an increasing trend with hardness, reaching 3.8 MPa·m^1/2.

In the concluding section of the thesis, the toughening and strengthening effects of crystal defect engineering are summarized and compared to other methods. Future work based on the results of this thesis is also proposed.

Date of Award	6 Jul 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Paul Kim Ho CHU (Supervisor)