Influence of Sintering Temperature on the Performance of Zirconia Ceramic Grinding Balls

Since the 1970s, Professor Gavrie from Australia discovered the toughening mechanism of zirconia ceramics, zirconia ceramics have found wide applications in various structural and functional ceramics. Zirconia ceramic grinding balls, as a highly used and versatile product within zirconia ceramics, are widely applied in industries such as ceramics, building materials, chemicals, coatings, electronics, machinery, food, pharmaceuticals, and cosmetics. Due to the excellent properties of zirconia, including high hardness, high strength, high toughness, wear resistance, corrosion resistance, and high density, zirconia ceramic grinding balls are not only extensively used for grinding zirconia ceramics but also for other applications such as electronic ceramic powders, magnetic material powders, high-tech structural and functional ceramic powders, daily ceramic pigments and glazes, chemical coatings, mechanical polishing powders, pharmaceuticals, and food powder grinding, where they provide unmatched grinding and dispersion effects, reducing impurities and improving grinding efficiency.

Sintering is a critical step in the production of ceramic products, as it plays a significant role in determining the microstructure and properties of ceramic materials. During sintering, the physical and chemical properties of ceramic bodies change, which ultimately influence the performance of the final product. One of the crucial process parameters during sintering is the determination of the sintering temperature. This study focuses on the impact of sintering temperature on the density and wear resistance of rolled zirconia ceramic grinding balls.

Preparation and Performance Testing of Zirconia Ceramic Grinding Balls1.1 Preparation of Zirconia Ceramic Ball BlanksZirconia ceramic grinding ball blanks were prepared using sub-micrometer 3% Y2O3-stabilized ZrO2 (molar fraction) with a median particle size of 3.75 µm as the raw material. The ball blanks were fabricated using a rolling forming method. The manufacturing process involved placing a certain amount of pre-made zirconia ball cores (with a size smaller than 0.5 mm) into a rolling mill and continuously rotating them at a constant speed of 40 rpm. A 0.3% polyvinyl alcohol water solution was sprayed onto the ball core surfaces, followed by adding zirconia powder to the mix. After rolling for 1 minute to ensure the firm attachment of the powder to the ball cores, the ball size gradually increased to about 7 mm. Spraying was stopped, and the ball blanks were further rolled for approximately 30 minutes to polish their surfaces. After stopping the machine, the resulting zirconia ceramic grinding ball blanks were smooth, round, and of good roundness. The blanks were allowed to air dry for 24 hours and then thoroughly dried in an 80°C oven. The density of the dried ball blanks was calculated based on the total volume determined by measuring the diameter of ten ball blanks and weighing their total weight. The calculated density of the dried ball blanks was 4.29 g/cm³.

1.2 Sintering Process for Zirconia Ceramic Ball BlanksThe zirconia ceramic ball blanks were divided into four groups, each subjected to different sintering processes. The sintering process involved heating from room temperature to the desired sintering temperature, with a heating rate of 100°C/h. Once the temperature reached 1000°C, the heating rate was reduced to 50°C/h, and the sintering temperatures for the four groups were set at 1450°C, 1500°C, 1550°C, and 1600°C. The samples were then cooled inside the furnace to obtain zirconia ceramic grinding balls with a diameter of approximately 6 mm.

1.3 Testing of Zirconia Ceramic Ball PropertiesDensity Measurement: The volume density of the sintered zirconia ceramic grinding balls was determined by measuring their dry weight and weight in water using a balance with a precision of 0.0001 g. The volume density of the wear-resistant ceramic balls was calculated using the Archimedes method.

Abrasion Rate Measurement: Approximately 1 kg of zirconia ceramic grinding balls from each group was selected for abrasion rate testing. These balls were placed in a polyurethane canister with an inner diameter of 220 mm and a length of 220 mm. Then, 500 mL of deionized water was added, and the mixture was ground at 60 rpm for 48 hours using a ball mill. After the test, the samples were washed with water, dried in an 80°C oven, and then weighed to calculate the abrasion rate, expressed as (m - mZ) / (m * 48), with units in 10^-6/h.

Results and DiscussionImpact of Sintering Temperature on the Volume Density of Zirconia Ceramic Grinding BallsFigure 1 shows the variation in volume density of zirconia ceramic grinding ball blanks prepared using the rolling forming method and sintered according to the specified sintering process as a function of sintering temperature. It is evident that as the sintering temperature increased, the volume density of the zirconia grinding balls gradually increased. Between 1500°C and 1550°C, there was a sharp increase in volume density, while between 1450°C and 1500°C and between 1550°C and 1600°C, the curve flattened. Clearly, at 1450°C and 1500°C sintering temperatures, the zirconia ceramic grinding balls did not fully sinter and densify, with volume densities around 5.7-5.759 g/cm³. After sintering at 1550°C, the zirconia ceramic grinding balls exhibited a significant increase in volume density, indicating substantial sintering and densification. The volume density reached 5.919 g/cm³, which is approximately 97.9% of the theoretical density (calculated as 6.109 g/cm³). At 1600°C, although the volume density slightly increased, the excessively high sintering temperature resulted in some grains growing abnormally (the influence of process conditions on the microstructure of zirconia ceramics will be discussed in a separate article).

Because the grain size of zirconia ceramics controls the potential for stabilizing the tetragonal phase transformation to the monoclinic phase during cooling to room temperature, the excessive growth of some grains at 1600°C results in a phase transformation to the monoclinic phase, reducing the content of tetragonal phase zirconia that contributes to wear resistance at room temperature. As a result, the wear resistance decreased, and the abrasion rate started to rise.

Conclusion(1) Zirconia ceramic ball blanks were prepared using the rolling forming method, and these blanks had smooth surfaces, good roundness, and high volume density. Relative volume densities of approximately 97% were obtained for the ceramics sintered at 1550-1600°C.(2) The wear resistance of zirconia ceramic grinding balls is mainly influenced by their volume density and the content of tetragonal phase zirconia capable of phase transformation. Under the conditions of this experiment, zirconia ceramic grinding balls manufactured using the rolling forming method and sintered at 1550°C exhibited excellent wear resistance, with an abrasion rate of 2.6 x 10^-6/h.