Wear Materials and Nano Composites

3.6. Wear Materials and Nano Composites

Normally, the ‘wear’ can be subdivided into two distinct categories :

A. Erosive Wear in which the particles in a gaseous or liquid medium strike a surface, and

B. Frictional Wear where two materials are in sliding contact.

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There are some important material properties for ‘wear applications’, such as : (a) Hardness, (b) Fracture Toughness, (c) Thermal Conductivity, (d) Chemical Inertness, and (e) Corrosion Resistance. There are many ceramic materials that possess these properties, which make them suitable for a

range of applications such as : (a) Orthopaedic Implants, (b) Thread Guides, (c) Seal Rings, (d) Valves, (e) Metal Extrusion Dies, and (f) Chutes in Material Handling Systems. Alumina ceramics have an excellent wear-resistance and it is also cost effective, i.e. higher wear-

cost index, and hence they are used in many applications as wear parts. Although there are a variety of other ceramic materials such as :

(a) Silicon Carbide (SiC), (b) Partially Stabilised Zirconia (PSZ), (c) Zirconia Toughened Alumina (ZTA), (d) Tungsten Carbide (WC), and (e) Silicon Alumina Oxy-Nitrides (SIALONS). These interesting materials also have superior wear resistance, but still alumina has been the

“material of choice” for many applications primarily because of its ‘Low Cost’ and ‘Ease of Fabrica- tion’.

For example in a comparative sense, an Alumina Seal costing Rs. 400 would cost approximately Rs. 1200 in Silicon Carbide and Rs. 1600 in Tungsten Carbide. However, standard alumina ceramics do not meet the more stringent requirements of ‘high wear resistance’ applications. As a consequence, the materials with increased wear performance, such as SiC, Sialons and ZTA, are now being routinely used as wear parts, despite their higher costs for many hi-tech applications.

Therefore, clearly, there is a ‘need’ that existed in the search of materials in order to bridge the gap between Low Performance-Low Cost and Hi-Tech-High Cost. There is a question of material strat- egy here, wherein one should look for newer materials that would perform properly at a reasonable cost. This has led to a new thrust in order to improve the wear resistance of alumina based ceramics. The toughness and wear resistance have been increased through the use of some materials such as :

(a) SiC Fibres, (b) Whiskers, and (c) Platelets or Particles. But there are certain ‘related’ problems like : (a) Health Issues related to Whiskers,

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133 (b) Anisotropy of Properties (i.e. axial dependence),

(c) Difficulties with Processing and Densification, and finally (c) Creation of some Defects.

The consequential effect is that the ‘increases in toughness’ are not always commensurate with the ‘increases in fracture strength’. That brings us to the newer development of nano-composite ceramics, which might be in the limelight in the search of ‘real’ materials.

3.6.1. Nano-Composite Ceramic Materials

A new family of materials called ‘Nano-Composites’ have enhanced mechanical properties. For the application as structural ceramic materials, a nano-composite represents a ceramic matrix composite with the grain size of the matrix in the range of several microns and containing a second phase of nano- sized (< 100 nm) grains.

During the past, many systems with matrix-inclusion have been investigated such as : (a) Alumina, (b) Magnesia, (c) Silicon Nitride, (d) Mullite, and (e) Cordierite as matrix materials. The second phase materials used as the ‘inclusion-phase’ are : (a) Silicon Carbide, (b) Silicon Nitride, (c) Titanium Nitride and (d) Metallic Particles. But, the system which has attracted more attention is ‘Alumina-Silicon Carbide’. An initial study shows that the mechanical strength for this new class of materials increases from

360-390 MPa to 1450-1500 MPa on addition of 5% SiC to an Al 2 O 3 matrix. In case of fracture tough- ness it is observed that it increases from 3.6 to 4.7 MPa.m 1/2 . The creep resistance, thermal shock resist- ance and hardness also seem to show some improvements [34].

However, there remains some difficulties in getting uniform distribution of nano-sized SiC in the alumina matrix. Some improvements are obviously possible by processing the material through the Attrition Milling route in order to achieve high density with a dense microstructure, as done for the nano particles of both silicon carbide and alumina, as depicted in the previous sections.

3.6.2. Nano Composite Alumina Ceramics

Both Al 2 O 3 and SiC powders were as-received commercially as already explained in the previ- ous sections. The processing route was through Attrition Milling, as also described for both silicon carbide and alumina. By the use of proper dispersing agents, the distribution of SiC was ensured within the alumina matrix, which was a very important aspect for the preparation of the nano composite materials for high performance applications. Some care has to be taken for the processing of nano composite materials in order to ensure a 'high degree of homogeneity'.

Although hot pressing technique has been used so far for the fabrication, i. e. sintering, of nano composites based on aluminous materials, only pressureless sintering method has been used [34], like it has been done in case of microwave sintering of high density alumina (see section 3.4.3.) [3], and in the processing of high density silicon carbides (see chapter 2) [4, 5], with nearly the theoretical density achieved in both these cases.

With the help of ‘sintering additives’ that allowed pressureless densification of nano-composites at industrially acceptable temperatures, a preliminary study on the sintering behaviour of Al 2 O 3 and some Al 2 O 3 -SiC systems was undertaken. The Al 2 O 3 material was fully dense on sintering in a normal

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furnace at 1540°C, whereas the Al 2 O 3 -4.5 vol% SiC nano-composite material required sintering at 1680°C in order to obtain almost full density, i.e. 97.5%.

However, the addition of 1.1 wt% Y 2 O 3 (99.9% purity) as a sintering additive had a significant effect with > 98.5% density achieved at temperatures as low as 1530°C. The TEM photo (not shown here), which has already been shown in Figures 3.7 and 3.8, indicate a fine-grained microstructure [34]. This type of inter-connected microstructure is very helpful. Although the observed densification and the microstructural aspects are quite encouraging, but a better microstructure can be still obtained for a better ‘wear’ property by a combining various types of ‘sintering additives’ in order to reduce the sintering temperature further as well as to get better abrasion-resistance. This was recently done quite extensively by an intercontinental group at Ireland [35].

O'Sullivan et al [35] found out that Al 2 O 3 -SiC nanocomposites, produced by industrially accept- able processes including pressureless sintering, had higher ‘wear’ resistence than alumina and, in turn, the nano-composites doped with a combination of different ‘sintering aids’ showed higher ‘wear resist- ance’ than undoped nano-composites. At lower SiC region, the ‘wear resistance’ increased with SiC content up to 3 vol% with only little improvements observed above this limit. In erosive wear tests, these workers found out that the doped materials had ‘wear resistance’ 30 times higher than that of alumina. It

has been proposed that nano-sized SiC particles located at Al 2 O 3 -Al 2 O 3 grain boundaries contribute significantly to increased wear resistance through pinning and strengthening of the grain boundaries, which is an interesting observation by this group [35]. Some more interesting investigations on the line of improving mechanical properties of alumina nano-composites are given below.

Although pressureless sintering is now popular for making alumina nano-composites with SiC, this work needs some mention, since the material was prepared differently than the conventional tech- nique. In this work, the hot-pressing temperature, matrix grain size, fracture mode and the distribution of

SiC were studied to reveal the effect of SiC inclusions on the microstructure of Al 2 O 3 . The Al 2 O 3 -SiC composite powder was prepared by the ‘precipitation’ method [36].

The nano-scale SiC particles were coated with Al 2 O 3 . After hot pressing, most of the nano-scale SiC particles were randomly located within the Al 2 O 3 grains. The inclusions of SiC raised the hot- pressing temperature, decreased Al 2 O 3 grain size, and inhibited the abnormal grain growth of Al 2 O 3 . The inter-granular fracture for Al 2 O 3 was transformed to the intra-granular fracture for Al 2 O 3 -SiC nano- composites due to the addition of SiC. The observed microstructure changes increased the mechanical properties of Al 2 O 3 -composites by 40% [36].

Recently, the wear and other mechanical properties of nano-composites have been increased by engineering the reinforcement of SiC platelets, so this work needs some mention where also a new technique was used for material preparation. In this work, BaO-Al 2 O 3 -2SiO 2 (BAS) glass-ceramic pow- ders were prepared by sol-gel technique. The SiC platelet reinforced BAS glass-ceramic matrix com- posites with high density and uniform microstructure were fabricated by hot-pressing. The effect of additional crystalline seeds on Hexagonal to Monoclinic phase transformation of BAS was studied [37].

The effects of SiC platelet content on the microstructure and mechanical properties of the com- posites were also investigated. The results showed that the flexural strength and fracture toughness of the BAS glass-ceramic matrix composites can be effectively improved by the addition of SiC platelets. The main toughening mechanism was crack deflection, the platelets pull-out and the bridging. The increased value of flexural strength is contributed to the load transition from the matrix to SiC platelets [37].

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135 This particular work is also special in that alumina was already toughened by stabilized zirconia.

In this work, Al 2 O 3 ceramic matrix composites toughened with 3Y-TZP (yttria-containing tetragonal zirconia polycrystals) and oriented hexagonal shape α-SiC platelets were fabricated by using slip cast- ing and cold isostatic pressing. The effect of platelet aspect ratio and volume fraction on fracture tough- ness, and mode and microstructure of the final composite were examined. The fracture toughness of the reinforced-composites was evaluated using indentation in four-point bending test. The scanning elec- tron microscopy and transmission electron microscopy were used to determine the microstructural de- tails and dominant toughening mechanisms, which occurred in these materials [38].

The toughness measurement tests and detailed observations of microstructures and fracture sur- face profiles have led to the conclusion that multiple-toughening behaviour via transformation toughen- ing, microcracking, crack deflection, load transfer and platelet debonding and pull-out, as well as ther- mal residual stresses have a significant contribution in improving the fracture toughness. A fracture toughness value as high as 11.2 MPa . m 1/2 was achieved for the materials sintered at 1600°C for 3 h with a platelet addition of 30 vol% [38].