| Dental ceramic material has been widely applied in the field of dental prosthesis due to its excellent aesthetic performance,excellent biocompatibility and good adhesion to teeth,and is considered to be one of the main directions of dental restorative materials in the future.Current all-ceramic materials commonly used in the clinic include zirconia ceramics,alumina ceramics,glass ceramics,etc.Although the mechanical properties of zirconia ceramics and alumina ceramics in dental ceramics are high,both of them have poor translucency and aesthetics,and are sticky.The connection performance is not as good as that of glass-ceramics,and they all belong to sintered ceramics.They consist only of crystal phases and do not have the ability to regulate the microstructure of glass-ceramics.Compared to zirconia and alumina ceramics,glass ceramics have a glassy phase inside,which has good acid-etching adhesion,good aesthetics,and great potential for microstructure regulation.Therefore,this research work will select clinically used lithium disilicate glass ceramics,starting from the regulation of crystal structure angle,improve the overall performance of the material,reduce early failure rate,improve the total survival rate,and explore a new way for the improvement of dental ceramic materials.AIM:This study selected lithium disilicate glass ceramics with good crystallization ability,translucency and strength and started from the regulation of crystal structure,by means of oral fatigue wear test machine,3D profilometer,scanning electron microscope,X ray diffraction,differential thermal analysis and nanoindentor,through the the crystal scale,direction control,improvement of heat treatment to further improve the overall performance.METHOD:1.To regulate different crystal sizes of lithium disilicate glass ceramics through the heat treatment:IPS e.max Press ceramics were used as the original material and were melted at 1450°C.To determine the crystallization characteristics of the as-prepared glass,differential scanning calorimetry(DSC)experiment was conducted.The first step at 616?for all specimens was designed to form lithium metasilicate phase.The second step was aimed to form lithium disilicate phase,in which specimens G1–G4 were heat-treated at 765?,810?,855?,and 915?,respectively.To achieve equilibrated crystallinity after the second step treatments,the annealing time was decreased with increasing annealing temperature.The pressable glass-ceramics and the ingots by heat treatment were two control groups.Scanning electron microscope and X-ray diffraction analysis were conducted.2.To analysis the effects of crystal size on lithium disilicate mechanical properties:six groups were tested using flexural strength test,nanoindentation test and toughness measurements.3.The effect of crystal size on the wear time characteristics of lithium disilicate ceramics was investigated.The wear experiment adopted a self-made oral fatigue wear tester with patent invention.The 10mm diameter stainless steel ball was fixed on the loading head of the fatigue test machine to simulate the tooth tip,and the loading force was set to 350N.From the six group of 8 randomly selected lithium disilicate ceramic specimens,the wear cycle frequency of 4HZ.In the current wear test,2.4×10~6 loading cycles were performed to simulate 10 years of clinical service.After every 300,000cycles,impressions on the occlusal surfaces of specimens were obtained using a silicone impression material.The impression surface was measured using a non-contact 3D white light profilometer.The wear volume loss area,wear rate and dynamic curve changes with the number of cycles were calculated.4.With the wear experiment,the surface roughness of lithium disilicate glass ceramics rises rapidly in the"running period"and is at a high level.After the"stable wear period",the surface roughness is basically stable.The average Sa value of G3ceramics with the middle crystal size is the smallest,and the micro morphology and the general morphology are smooth.5.The influence of repeated pressing on microstructure and mechanical properties of lithium disilicate ceramic:the original blocks were used for first pressing and repeated pressing,which were comparatively analyzed with the G3 group of medium size through heat treatment.The flexure strength was measured by three point bending method.The phase analysis and microstructure observation of X ray diffraction were carried out.6.control crystal directions of lithium disilicate ceramic through equal channel angular pressing(ECAP)design principle:different diameter graphite die casting were designed and were compared with common heat-pressed lithium disilicate ceramic.Three groups were tested using indentation test and toughness measurements.And microstructure was analysed.RESULT:1.There are two exothermic crystallization peak temperatures of lithium disilicate glass ceramic glass.The peak temperature is 616?and 810?respectively.The first peak is related to the glass transition to lithium metasilicate,and the second peak is related to the formation of lithium disilicate phase.The starting and ending temperatures of the second peaks are 765?and 855?respectively.The main crystal phase of the six groups is lithium disilicate crystal,and it also contai ns a small amount of lithium metasilicate phase and lithium phosphate phase,and the crystallinity of glass ceramics is basically the same.The average width of crystals from G1 to G4increased from 0.19 to 0.35?m,while the average crystal length ranged from 0.61 to2.75?m,and the average length and aspect ratio increased significantly(from 3 to 8).G4,GC-P and GC-O exhibit similar crystal lengths(about 3?m),but the latter have larger crystal widths and smaller crystal aspect ratios(P<0.05).2.When the temperature reaches 855,the bending strength increased from(298.41±31.10)MPa to(361.22±21.20)MPa,reached the maximum value and becomes stable,and the hardness,modulus of elasticity and fracture toughness reach the highest and begin to decrease.Lithium disilicate glass ceramics with a medium size of(0.92±0.14)?m length have excellent mechanical properties.Lithium disilicate glass ceramics with small crystal scale(0.61±0.11)?m showed lower flexural strength,while lithium disilicate glass ceramics with large crytal scale(3.22±0.41)?m had lower hardness,elastic modulus and fracture toughness.3.During the whole cyclic loading process,the volume loss of G1-GC-O group ceramics increased constantly and showed the changes in different stages.In group G1,G2,G4,GC-P and GC-O,before 30×10~6 cycles,the volume loss was increased rapidly and the wear rate was high,and the wear rate was stable.During 6 0~240×10~6cycles,the volume loss increased slowly,and the wear rate of all circular nodes was not statistically different(P>0.05).In the G3 group,the volume loss was increased rapidly and the wear rate was high before 60×10~6 cycles.The 90~240×10~6 cycles were the stable wear period,which showed that the volume loss increased slowly,and the wear rate of all circular nodes was not statistically different(P>0.05).After 240×10~6 cycles,the amount of wear in group G3(0.37±0.12)mm~3 was lower than that of other 5 groups,followed by group G1(0.56±0.12)mm~3 and G2 group(0.65±0.18)mm~3,and there was no statistical difference between the two groups(P>0.05).The wear of group G4(1.01±0.25)mm~3 was higher than that of the former three groups.The highest wear rate was GC-P(1.67±0.24)mm~3 and GC-O group(1.67±0.14)mm~3,and there was no significant difference in the wear volume between the latter two groups(P>0.05).4.The microstructure and surface roughness of the six groups show different characteristics in different wear stages.The evolution mechanism of the micromechanism with time is in agreement with the evolution rule of the macroscopic wear curves of the two materials in Experiment three.The surface roughness of six groups of lithium disilicate glass ceramics rapidly increased at the run in period,and at a high level.After entering the"stable wear period",the surface roughness remained basically unchanged.Sa of medium crystal size group G3 was smaller than that of G1,G2 group,GC-P group and GC-O.After 240×10~6 cycles,the G1 group appears on the surface of irregular pits,widely distributed wear uniform;group G2 pit area is similar to that of group G1,group G3 pits relatively smooth worn surface is shallow;compared to the former three groups,group G4 group with large pits area,the wear scar is deep,scattered in shallow pits;and GC-P group GC-O pit is deep,large area,a large number of local scar formation,hill tall class like structure,uneven surface.5.The crystal phase of the first die-casting and two die-casting groups,as well as the lithium disilicate crystals in the G3 group,formed a rod like structure,but the crystal size was obviously different.The G3 group showed a short rod like crysta l(0.92±0.14)?m,and the first die-casting group showed a rod like crystal(3.22±0.41)?m,and the crystal scale of the two die-casting group increased to(4.42±0.88)?m.The first group of casting glass ceramic crystallinity is about 64%±4.8%,G3 group of glass ceramics crystallization is about 64%±1.8%times,casting glass ceramic group crystallinity is about 48%±4.3%,the crystal volume fraction was significantly lower than that of the first casting group and G3 group;lithium disilicate glass ceramics mechanical properties after hot pressing in two repeatedly significantl y drop.The bending strength of lithium disilicate glass ceramics decreased from(342.28±37.85)MPa to(261.65.28±41.64)MPa after repeated hot pressing.6.Through equal channel angular extrusion method can control the direction of the two more lithium disilicate ceramic die cavity and the diameter of the tube is helpful to improve the direction of the crystal.Reducing the diameter of different crystal orientation after the hardness value is higher than that of the conventional heat group hardness had significant difference(P<0.05).However,there is no significant difference between hardness values in the direction perpendicular to and parallel to the crystal flow direction.The fracture toughness of the different crystal orientation was lower than that of the conventional thermal die-casting group,and the difference was statistically significant(P<0.05).CONCLUSION:Higher heat treatment temperatures help to improve the size of the crystals.Under simulated oral masticatory environment,the wear volume of lithium disilicate ceramics with different crystal sizes showed two obvious characteristic wear stages with running time and stable wear period.The bending strength,hardness and fracture toughness of the medium crystal ceramics with(0.92±0.14)um length are the highest.The lithium disilicate glass ceramics with a(0.92±0.14)um length medium scale crystal have excellent wear resistance,followed by lithium disilicate glass ceramics with small crystal size,and the wear resistance of large scale ceramics is poor.It is not clinically suitable for repeated thermal pressure to change the mechanical properties and the microstructure of the IPS E.Max Press sample.The way of equal channel angular pressing can control more directional lithium disilicate ceramics,but we need to further optimize the die casting way and improve the mechanical properties. |
PDF Full Text Request