yttria tetragonal zirconia polycrystal ( y - tzp ) ceramics serve as an upcoming material for engineering applications and are also considered important for restoration medicine . the excellent mechanical characteristics such as high strength and fracture resistance are attributed to the ceramic 's unique ability known as transformation toughening , a stress - induced phase transformation from tetragonal ( t ) to monoclinic ( m ) [ 1 , 2 ] . the stabilizing effect of yttria ( y2o3 ) makes it possible for y - tzp ceramic to be processed in the metastable tetragonal ( t ) structure . this is essential since the retention of the ( t ) phase at ambient temperature allows it to transform to the monoclinic ( m ) structure under external applied stress [ 3 , 4 ] . despite its outstanding properties , kobayashi et al .   discovered that the y - tzp ceramics suffer a serious limitation for applications near 250c in moist environment . the findings revealed that the ceramic can suffer a slow , tetragonal to monoclinic phase transformation at the samples surface in a humid atmosphere , followed by microcracking and a major loss in strength known as low temperature degradation ( ltd ) [ 610 ] . since then , experiments have been conducted with y - tzp in an attempt to understand the basic micromechanisms of the ageing - induced ( t)-(m ) phase transformation and to suppress the ltd phenomenon [ 1119 ] . based on research findings , the addition of ceramic oxides ( mgo , al2o3 , zno , cao , and ceo2 ) helps overcome if not prevent the low temperature degradation occurrence in y - tzp ceramics [ 9 , 2024 ] . ceo2 is generally used to stabilize the tetragonal phase of zirconia and is also known to increase the sintering of glass ceramics and strength and thermal stability . ce - tzp in contrast to y - tzp has complete resistance to ltd and higher toughness . enhancing the moderate ce - tzp properties would result in the material becoming a primary candidate for biomedical applications which has the requirements in terms of phase stability and long - lasting characteristics in the human body . fischer and stawarczyk found that the ce - tzp / al2o3 nanocomposite offers superior mechanical properties compared to conventional y - tzp . also , when compared to conventional y - tzp the flexural strength is in the same range whereas the fracture toughness increases with the addition of ceria . investigating the effect of ceria addition both supported the statement that ceria increases the fracture toughness of y - tzp [ 28 , 29 ] . kuroda et al .   have shown that several reactions occur during sintering with the addition of cuo which can be beneficial or detrimental for densification of y - tzp . more recently , tribological studies on 3  mol% y - tzp ceramics showed that the addition of 1.8  mol% of cuo led to a reduction of friction coefficient from 0.2 to 0.6 under dry sliding conditions , while another study revealed that the ferromagnetic behaviour of y - tzp with enhanced coercivity could be achieved by doping 0.3  mol% nio in the zirconia matrix . the susceptibility of y - tzp to low temperature degradation ( ltd ) is a shortcoming for a material that has excellent mechanical properties and biocompatibility . the addition of cerium oxide , ceo2 , makes the material inert to ltd but is counterbalanced by modest strength and hardness , making its usage in biomedical applications limited and difficult . in order to overcome this limitation , the amount of ceo2 addition to y - tzp is altered along with the sample preparing factors such as sintering methods and temperatures , mixing methods , and the purity of powders used . two commercial type powders were prepared for this experiment : the 3  mol% of yttria - stabilized zirconia powder manufactured by kyoritsu ltd . , japan , under the code name of kz-3yf as the base powder and cerium oxide , ceo2 , manufactured by sigma aldrich , as the dopant . three different compositions of cerium oxide were mixed with y - tzp , that is , undoped 0.3  wt% , 0.5  wt% , and 1.0  wt% . the powders were mixed using the wet milling method , using zirconia balls as mixing media and ethanol as the mixing medium . the slurry was oven dried at 60c sieved , and the powder was uniaxially pressed at 0.3  mpa into discs and rectangular bars . these samples were then sintered under atmospheric condition at a rate of 10c / min , with temperatures ranging from 1250c to 1450c , and a holding time of 2 hours before cooling down to room temperature . the sintering temperatures 1250c to 1450c are used for this research as it is sufficient to retain the tetragonal phase of y - tzp . besides that , this temperature range is the optimum temperature range to obtain the high mechanical properties of y - tzp . disc samples are polished on a polishing machine with grinding papers of different roughness ( 120 , 240 , 600 , and 800 cc - cw ) and finally are polished using diamond paste of 6 microns and a polishing cloth . the bulk density of the sintered samples was measured based on archimedes ' principle , using the water immersion method with a standard mettler toledo balance ag204 densimeter . polished samples were used to determine vickers 's hardness and fracture toughness ( kic ) using vickers 's indentation method . an indentation with a force of 9800 the fracture toughness was calculated for the measured value obtained using niihara 's   formula as stated below : 
 ( 1)kic=0.016(e / h)1/2c3/2,kic being the fracture toughness ( mpam ) ; e is young 's modulus ( gpa ) , h the vickers hardness ( gpa ) , and c the crack length ( m ) measured from the center of indentation . the microstructure of the material was evaluated using sem ( scanning electron microscope ) whereby the grain structures and grain sizes were determined using the average grain intercept method . the average grain intercept method is used to measure the grain size of a material by drawing randomly positioned line segments on the micrograph , counting the number of times each line segment intersects a grain boundary , and finding the ratio of intercepts to line length . figure 1 shows the influence of various ceo2 additions to the bulk density of y - tzp . the sintering temperature acts as a controlled variable ; ceo2-y - tzp samples are sintered over a temperature range of 12001450c . the addition of ceo2 was beneficial in aiding densification as the increase in density was proportional to the increasing sintering temperature . the figure shows that samples with ceo2 content > 0.3  wt% achieved much higher values of densities as compared to samples of lower amount and undoped samples . the highest value of density was recorded by the samples with 0.5  wt% ceo2 , which is approximately 97% of the theoretical value ( 6.09  g / cm ) . a significant reduction in density values was observed at higher temperatures , a phenomenon probably attributed to the grain growth . besides that , the decrease could also be associated with agglomeration of the ceo2-y - tzp particles during the preparation and sintering processes . the effect of sintering temperatures and ceo2 doped samples on the vickers hardness of y - tzp is shown in figure 2 . the undoped sample recorded a hardness value of 9.6  gpa ( 978.9  hv ) . when sintered at 1250c , the doped samples showed higher hardness values at 1250c but was still below the theoretical value . the highest value recorded was 13.2  gpa ( 1346  hv ) at a sintering temperature of 1400c , before facing a drop in value with further sintering at 1450c . overall , from observation it was found that samples with 0.5wt % and 1.0  wt% dopant addition sintered at temperatures > 1300c achieved hardness values higher than the theoretical vickers hardness of y - tzp . the decrease in hardness at high temperatures ( > 1400c ) can probably be associated with the increased proportion of transformable tetragonal phase and associated pseudoplasticity . the influence of the addition of ceo2 on young 's modulus of the sintered sample is depicted in figure 3 . at 1250c , all doped samples reached almost the theoretical value of young 's modulus ( 210  gpa )   whereas the undoped y - tzp was low ( ~177  gpa ) . the 0.3  wt% ceo2-y - tzp doped sample shows a decline in e value from 1350c onward , and in contrast 0.5  wt% ceo2-y - tzp kept increasing up till 1450c and attained the highest value of ~211  gpa . an inversly proportional relationship exists between the bulk density and young 's modulus as a lower density results in a higher elasticity in the material . it can be inferred that the bulk density is an important parameter governing the matrix stiffness of y - tzp . similar results were seen in work from other researchers [ 37 , 38 ] . the influence of the varying amounts of ceo2 and various sintering temperatures on the fracture toughness was seen in figure 4 . it was clearly seen that the addition of ceo2 was beneficial in enhancing the fracture toughness of y - tzp . at 1250c , the undoped sample only reached the fracture toughness value of 4.5  mpam while doped samples achieved the higher toughness ranging from 4.8  mpam to 5.5  mpam . all samples showed improvement in toughness when sintered 1300c , except for the 0.3  wt% doped samples , which in contrast displayed a decreasing trend up till 1450c . sintering at higher temperature ( 1300c ) seems to have an outstanding effect on the fracture toughness of y - tzp , especially with the addition of 0.5  wt% ceo2 . the highest value of fracture toughness recorded was 6.4  mpam for 0.5  wt % ceo2 addiition at 1400c . the high fracture toughness of ceo2-y - tzp sample could be attributed to the resistance of the material to crack propagation and also the grain growth . figures 5 and 6 illustrate the effect of the addition of 0.3  wt% and 0.5  wt% ceo2 on the microstructure of the ceramic . the 0.5  wt% doped sample revealed a microstructure that is more homogenous than the microstructure of the 0.3  wt% ceo2 doped specimens . the average grain intercept is determined from two of the best samples ( polished ) with different dopant content sintered at 1400c . it is seen that the samples containing 0.3  wt% ceo2 have a bigger grain size with an average value of 2.13  m whereas the samples with 0.5  wt% ceo2 have a smaller grain size value of 1.586  m . the current work shows that adding ceo2 to y - tzp has proved to be beneficial . the bulk density was found to be higher with a value of ~5.9  g / cm ( ~97% of the theoretical density ) , with the addition of up to 0.5  wt% ceo2 when sintered at 1400c . all samples with 0.5  wt% ceo2 produced the highest values for mechanical properties ; the vickers hardness was increased to ~13.2  gpa ; young 's modulus was enhanced to ~211  gpa ; fracture toughness was increased to ~6.4  mpam . also , sintering temperature had an important role in determining the mechanical properties and microstructure of the material as most increment in values occurred at temperatures  1300c .