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|New Alumina Based Ceramic Materials for Abrasive and Wear Applications|
|David O’Sullivan, Materials Ireland Research Centre, University of Limerick, Ireland|
Ceramic Wear Materials
In general, wear can be divided into two categories, erosive wear, in which particles in a liquid or gaseous train strike a surface, and frictional wear where two materials are in sliding contact. Important material properties for wear applications include hardness, fracture toughness, thermal conductivity, chemical inertness and corrosion resistance. Many ceramic materials possess these properties, making them suitable for a range of applications including orthopaedic implants, thread guides, seal rings, valves, metal extrusion dies and chutes in materials handling systems.
Alumina (Al2O3) ceramics have an excellent wear-cost index and are used in many applications as wear parts. Although other ceramic materials, such as silicon carbide (SiC), partially stabilised zirconia (PSZ), zirconia toughened alumina (ZTA), tungsten carbide (WC) and sialons, have superior wear resistance, Al2O3 has been the material of choice for many applications primarily because of its low cost and ease of fabrication. For example, an Al2O3 seal costing €10 would cost approximately €30 in SiC and €40 in WC. However, standard Al2O3 ceramics do not meet the more exacting requirements of high wear resistance applications. As a result materials with increased wear performance, such as SiC, sialons and ZTA, are now being routinely used as wear parts, despite their higher costs and over-sophistication for many applications.
A need exists for materials that bridge the gap between low performance/low cost and over-sophistication/high cost. New materials should perform adequately at a reasonable cost and this has led to research to improve the wear resistance of Al2O3 based ceramics. Toughness and wear resistance have been increased through the use of SiC fibres, whiskers, platelets or particles but there are associated problems including health issues relating to whiskers, anisotropy of properties and difficulties with processing and densification as well as the creation of defects. As a result increases in toughness are not always accompanied by increases in fracture strength. A new breakthrough has been the development of nanocomposite ceramics.
Nanocomposite Ceramic Materials
Nanocomposites are a new family of materials with enhanced mechanical properties. As applied to structural ceramic materials, a nanocomposite describes a ceramic matrix composite with matrix grain size of the order of several microns and containing a second phase of nanosized (< 100 nm) grains. Over the past number of years many matrix-inclusion systems have been studied including Al2O3, Si3N4, MgO, mullite and cordierite as matrices and SiC, Si3N4, TiN and metallic particles as the inclusion phase. The most studied system has been Al2O3-SiC.
Initial studies reported spectacular improvements in mechanical properties for this new family of materials with strength increases from 350 to 1520 MPa on addition of 5 vol% SiC to an Al2O3 matrix. Significant increases were also claimed for fracture toughness (3.5 to 4.8 MPam½), hardness, creep resistance and thermal shock resistance. However, subsequent research, including a project involving three Euroceram partners (BCRC [B], University of Limerick, [IRL] and the Université de Valenciennes [F]), while generally obtaining property improvements, failed to repeat the spectacular levels of strength improvements.
Although large strength and toughness improvements were not obtained, important outputs from the project were that significant improvements in wear performance and creep behaviour could be achieved on addition of 5 vol% SiC to an Al2O3 matrix. However, many obstacles remained to be overcome in order to exploit these materials. These included difficulties in obtaining homogeneous dispersions of nanosized SiC in the Al2O3 matrix and the requirement for industrial friendly processes to be developed: use of commercially available and competitively priced powders, powder processing in water, spray drying, and most importantly, the requirement for densification to be achieved through pressureless sintering.
Nanocomposite Alumina Ceramics for Advanced Technical Applications (NACATA)
A recently completed EU funded project (NACATA) involving the same three Euroceram partners as well as industrial and academic partners in the UK, Germany, Ireland, Italy and Austria successfully addressed these obstacles and developed Al2O3-SiC nanocomposite materials for applications as industrial wear parts and abrasive grits.
Commercially available Al2O3 and nanosized SiC powders were sourced and processing routes were developed for these powders which involved effective dispersion of the SiC powder through pH control and the use of dispersion agents. Attrition and ball milling were used to thoroughly mix the slurries, which were spray dried on a pilot industrial scale.
The development of pressureless sintering for ceramic nanocomposites was a significant breakthrough as, up to this point, it had only been possible to densify nanocomposites by using pressure techniques (hot pressing or HIP) because the presence of nanosized SiC significantly inhibits densification.
Sintering additive systems were developed that allowed pressureless densification of nanocomposites at industrially acceptable temperatures. The sintering behaviour of Al2O3 and some Al2O3-SiC systems are shown in figure 1. The Al2O3 material was fully dense on sintering at 1550°C whereas the Al2O3 – 5 vol% SiC nanocomposite (NC in figure 1) required sintering at 1700°C in order to obtain full density.
Addition of 1 wt% Y2O3 (NC-Y) as a sintering additive had a significant effect with >99% density achieved at temperatures as low as 1550°C. However, microstructural examination of this system showed an inhomogeneous microstructure containing long elongated grains (figure 2a). Although homogeneous equiaxed grains were observed in the microstructure of the nanocomposite doped with 1 wt% MgO (NC-Mg) (figure 2b), this material was difficult to densify with 98% density achieved only after sintering at 1650°C. Using a combination of Y2O3 for ease of sinterability and MgO for homogeneity of microstructure resulted in a material (NC-Mg,Y) with acceptable density (98%) after sintering at 1550°C and a homogeneous equiaxed microstructure (figure 2c).
Wear Behaviour of Al2O3 – SiC Nanocomposites
The wear behaviour of nanocomposites was investigated by three methods: wet erosive wear, polishing wear and pin-on-disc wear.
WET EROSIVE WEAR
The erosive wear behaviour of Al2O3 and nanocomposites doped with oxide sintering additives was assessed. Addition of 3 vol% SiC to Al2O3 significantly improved wear resistance by a factor of 8 (figure 3) and no further improvement in erosive wear resistance was observed on increasing the SiC content above this value. The erosive wear resistance of the 5 vol% SiC nanocomposite was further increased by adding 1 and 2 wt% Y2O3 (NC-Y and NC-2Y in figure 4). In terms of mixed doped nanocomposites, MgO + Y2O3 (NC-Mg,Y) had the lowest wear rates. Both NC-2Y and NC-Mg,Y materials had homogeneous microstructures (figure 2)
The wear surfaces of a number of materials are shown in figure 5. Significant intergranular fracture associated with complete grain removal during wear testing can be seen in the wear surface of the Al2O3 material (figure 5a). However, in the nanocomposite a smoother surface with less intergranular fracture was observed (figure 5b) consistent with a slower material removal rate. This observation correlates with the recorded change in fracture mechanism from intergranular in Al2O3 flexural strength specimens to transgranular in nanocomposites. The change in fracture mechanism is associated with better cohesion of Al2O3 – Al2O3 grain boundaries due to their pinning by SiC particles.
In the nanocomposite doped with 1 wt% Y2O3 the wear surface was smoother still with no intergranular fracture. In general, lower erosive wear rates were associated with smoother wear surfaces.
The effect of grinding and polishing on the surface roughness of three materials (Al2O3, NC and NC-Y) was evaluated. The surface roughness was measured on ground samples and then every 15 minutes during 45 minute cycles of polishing using 25µm, 8µm and 3µm diamond slurries. The results are shown in figure 6.
Polishing for 15 minutes with 25 m slurry sharply reduced the surface roughness of the nanocomposites compared to Al2O3. As would be expected, a smoother surface resulted from polishing with 8µm and 3µm slurries. The study showed that nanocomposites had a better surface finish than Al2O3 after each polishing step due to suppression of grain pullout resulting from grain boundary strengthening by SiC. Y2O3 doping further improved the surface finish of the nanocomposite probably due to a contribution of Y2O3 and/or grain boundary glass phases to grain boundary strength.
Pin-on-disc (POD) wear experiments were carried out in wet and dry conditions using nanocomposite pins on a standard alumina disc. The results from dry tests showed that the nanocomposite containing 1 wt% SiC and 1 wt% MgO had the best wear performance. However, the results suggested that a build up of wear debris in the groove probably acted as a free abrasive masking any effect due to wear between the pin and the disc. Wet tests were carried out using pins prepared from industrially sintered spray dried nanocomposites containing Y2O3 sintering additive. The general trend indicated that wear resistance increased as the SiC content increased. The lowest wear results were obtained for an Al2O3 – 1 wt% SiC pin containing 1 wt% MgO + 1 wt% Y2O3.
In the NACATA project, Al2O3-SiC nanocomposites, produced by industrially friendly processes including pressureless sintering, had higher wear resistance than alumina and, in turn, nanocomposites doped with sintering aids had higher wear resistance than undoped nanocomposites. At low SiC contents the wear resistance increased with SiC content up to 3 vol% with only little improvement observed above this value. In erosive wear tests, doped materials had erosive wear resistance up to 30 times higher than alumina. Materials prepared under industrial pilot scale conditions had wear properties similar to those prepared and sintered in laboratory conditions. Low wear rates were associated with smooth surface finish for grinding and polishing conditions. It is proposed that nanosized SiC particles located at Al2O3-Al2O3 grain boundaries contribute significantly to increased wear resistance through pinning and strengthening of the grain boundaries.
The results from the NACATA project point to a process and product that are very encouraging for the industrial partners. Plans are in place to scale-up the process further and to assess the products obtained with a view to commercialisation in the short to medium term.
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