**1. Introduction**

Materials with high contact damage resistance are extensively required in aerospace and aircraft, vehicle industry, microelectro-mechanical systems and devices, cutting tools and bulletproof vests [1, 2]. An approach for improved resistance to surface contact damage is to design surface gradations in composition, microstructure and elastic and/or plastic properties [2–4]. Such design provide effective means to enhance materials contact damage resistance through redistribution of thermal and/or mechanical stresses, elimination of interface-induced stress concentrations and reduction in the local crack driving force [5–8]. Nature is a master in the design of sophisticated hierarchical structured materials which provide excellent damage resistance [9]. A typical example is the material structural design principle found in a fish armor [10]. In response to predatory threats, fish are protected by armor scales consisting of four distinct reinforcing layers of organic/inorganic nanocomposites with hardness and modulus decreasing gradually from the outer to the inner layers. The juxtaposition of multiple reinforcing composite layers and the gradations, both in microstructure and mechanical properties within and between material layers, provides a more compliant protective mechanism than the monolithic counterpart [10].

Studies of the effects of microstructural, compositional, and property gradients on the overall elastic–plastic response under contact loading are an area of great interest, and much progress has been achieved in the fundamental understanding of graded surface damage resistance [2]. However, systematic investigations of the multilayered hierarchical structure on indentation response, contact damage resistance and contact surface failure of graded ultrafine-grained (UFG) metal, in particular with regards to the structural multilayering and grading, have not been investigated by multiscale experimental and computational approaches and the mecha-

Improving Contact Load-Bearing Resistance of Ultrafine-Grained Materials…

http://dx.doi.org/10.5772/intechopen.72197

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nism of the contact load-bearing response in these situations is also largely unknown.

load-bearing capacities for engineering applications.

a rectangular bar with dimensions of 5 × 5 × 90 mm<sup>3</sup>

**2. Experimental procedures**

lithic NG Ti has been given in [22].

In this Chapter, we focus on the contact load-bearing response and surface damage resistance of MHSed Ti relative to monolithic nanostructured Ti. Through experimental investigations and computational simulations of local stress and strain distributions, the mechanism of the contact load-bearing response of MHSed Ti is explored. These results provide clear evidence of improved contact load-bearing capacities through structural multilayering and grading. Such information is of practical value for the design of UFG materials with excellent contact

The MHSed Ti was produced by the following experimental procedures. A commercial Ti plate (Grade 2) with 36 mm in thickness was cryogenically rolled to 5 mm with per reduction of ∼2 mm. The detailed microstructure characterizations of the cryorolled Ti have been given elsewhere [22]. The cryorolled workpiece then was cut parallel to the rolling direction (RD) to

rectangular bar was subjected to SMAT. The SMAT process was performed in a low vacuum condition using hardened stainless steel balls (8 mm in diameter) at a vibration frequency of 50 Hz for 60 min. The detailed MHS process can be found in [20]. The production of mono-

**Figure 1a** shows a schematic illustration of the nanoindentation and contact load-bearing testing. Nanoindentation experiments were carried out at ambient temperature using an UMIS indentation system with a Berkovich diamond tip at a strain rate of 5 × 10−2 s−1 and a maximum load of 20 mN. Before testing, the cross-sectional surface was polished to 0.5 μm diamond suspension finish. The values of the nanoindentation hardness and modulus quoted here were the average of 10 measurements on the cross-sectional surface. Before Vickers microhardness and load-bearing testing, artifacts on the surface caused by MHS process were carefully removed by polishing to 0.5 μm diamond suspension finish (removal thickness < 2 μm). Vickers microhardness testing was conducted using a microhardness tester (FM 700) under a load of 0.5, 1, 3, 5, 10 N on the MHSed surface at more than 10 points and the average values were reported here. Load-bearing testing was conducted with a spherical tungsten carbide (WC) indenter with diameter of 1.5 mm in ambient conditions. The WC indenter had an elastic modulus of 640 GPa and a Poisson's ratio of 0.26. The indenter came into contact with the specimen surface and was loaded to a maximum load of 1000 N at a loading rate of 1000 N/s.

. Subsequently, one lateral surface of the

Inspired from the material structural design principle discovered in natural/biological systems, materials scientists have generated enormous interest in replicating natural/biological structures with excellent damage resistance than their conventional counterparts. Over the past 2 decades, significant progress has been made in synthesis and fabrication of materials with graded properties over multiple length scales. Elastically graded materials (EGMs), where the materials have gradient in elastic modulus as a function of depth beneath the surface, were synthesized by controlled infiltration of aluminosilicate or oxynitride glass into polycrystalline ceramic matrix, which offered superior resistance to contact damage than either constituent ceramic matrix or glass [11–13]. Plastically graded materials (PGMs) were produced by increasing or decreasing the grain size within the nanocrystalline or microcrystalline range to create a linear gradation of yield strength as a function of depth below the material surface according to classical Hall–Petch effect [3]. The benefit of the gradient effect on the stress–strain and deformation response under normal indentation have been demonstrated by analytical [14], computational [15–17] and experimental studies [18, 19].

In our recent work [20], we extended the EGM/PGM concept to design a multilayered hierarchical structure (MHS) on Ti. By the application of Surface Mechanical Attrition Treatment (SMAT) [21] to cryorolled Ti, a three-layered structure formed consisting of an outer amorphous/nanocrystallite (A/NC) layer, an inner nanograined (NG) layer and ultrafine-grained (UFG) core [20]. Nanoindentation through the cross-section of the multilayered hierarchical structured (MHSed) Ti revealed a gradual decrease in hardness and modulus within and between each successive structural layer [20]. These properties correlate with the microstructure characteristics and the design principle found in natural systems, such as fish armor [10]. The work hardening of the MHSed Ti was improved largely by such structural design [20]. Moreover, the gradations in structure and properties, pore and crack-free nature and the inherently damage tolerant top A/NC layer of MHSed Ti are expected to benefit the contact deformation and damage resistance of MHSed Ti.

Studies of the effects of microstructural, compositional, and property gradients on the overall elastic–plastic response under contact loading are an area of great interest, and much progress has been achieved in the fundamental understanding of graded surface damage resistance [2]. However, systematic investigations of the multilayered hierarchical structure on indentation response, contact damage resistance and contact surface failure of graded ultrafine-grained (UFG) metal, in particular with regards to the structural multilayering and grading, have not been investigated by multiscale experimental and computational approaches and the mechanism of the contact load-bearing response in these situations is also largely unknown.

In this Chapter, we focus on the contact load-bearing response and surface damage resistance of MHSed Ti relative to monolithic nanostructured Ti. Through experimental investigations and computational simulations of local stress and strain distributions, the mechanism of the contact load-bearing response of MHSed Ti is explored. These results provide clear evidence of improved contact load-bearing capacities through structural multilayering and grading. Such information is of practical value for the design of UFG materials with excellent contact load-bearing capacities for engineering applications.
