The Tribological Behavior Of Veneering Porcelain Of The Co-cr Alloy PFM Crown In Simulated Oral Environment

Veneering porcelain belonging to glass ceramic, with excellent translucency andbiocompatibility, has been widely used in clinic for many years. Generally, it often befused to metal base produced PFM crown or be sintered to ceramic core formed bilayerfull ceramic crown. Veneering porcelain can improve the aesthetic effect of the base crownand make the restoration more lifelike in appearance. These prostheses not only restore thestomatognathic system masticatory function, but also reshape the maxillofacial naturalappearance, given the dual treatment of the patient’s physiological and psychological.However, because of the ceramic material inherent brittle, the wear will be happen in theveneering layer easily during the process of repeatedly chew movements, and also themicrostructure of veneering porcelain may be damage, which can lead to crack nucleationand propagation and even can cause fracture. All the above-mentioned are becomingurgent and important issues in dental prosthodontics.According to mechanical tribology theory, the friction and wear behavior of the material should comply with the tribological characteristics of time,which means that thewhole wear process is a dynamic development process with time, and should contain threecharacteristics of wear stages:"running-in stage","stable wear stage" and "severe wearstage". While the previous studies of wear performance of dental ceramic are mostly justthrough a fixed test protocol, ignoring the three characteristics of wear stages of wearbehavior. Thus the prior result and conclusion has certain limitation. In order to fullyunderstanding the wear behavior of dental ceramic in the different process of wear stage,this research will based on the mechanical tribology theory, with the principle of“comprehensive evaluation time characteristic of the wear behavior theory”, to researchthe occurrence and development of wear behavior of the vennering porcelain. Since theveneering porcelain of the PFM crown is similar with full ceramic crown’s veneeringporcelain in composition and mechanical properties, the study intended to choose theveneering porcelain of co-cr alloy PFM crown as the research object.AIM:By the finite element stress analysis(FEA) and the Thermal cycling and mechanicalloading (TCML) method, explore the tribological characteristics of ceramic materialsunder different stress conditions, in order to master the role of wear development, the wayof microstructure evolution, and the influence of stress in the entire wear process. Thisresearch can also providing some experimental basis for design and use of ceramicrestoration properly, as well as further improving the mechanical stability of veneeringporcelain.METHOD:1. To evaluate stress condition of different wear degree of veneering porcelain byFEA: According to the indenter’s diameter and loading,divided the specimen into3groups:4mm diameter350N group;10mm diameter350N group;10mm diameter150N group.Analysis the stress magnitude and distribution of veneered layer of PFM under differentwear levels.2. Research for the tribological characteristics of veneered layer of PFM in simulatedoral environment: Divided the standard form of mandibular first molar co-cr alloy PFM crowns into3groups according to the indenter’s diameter and loading:4mm diameter350N group;10mm diameter350N group;10mm diameter150N group. Each ofspecimens will undergo TCML. Measureing the wear area of veneering porcelain surfaceover time by3D Profiling, draw the tribological time characteristic curve of ceramic indifferent stress groups.3.The study of the microscopic structure and related physical parameters of veneeringporcelain in different wear stage: Explore the wear surface and subsurface micro-structure,surface roughness in each wear stage respectively by SEM and3D profiling. And comparethe fracture loading of the veneering porcelain between different stress groups byuniversal testing machine.RESULT:1. The FEA showing: With the wear level going deeper, the veneering surface weararea increases, and the average thickness of the ceramic layer decreases, while the peakstress decreases. Among the different loading condition, the peak stress and its distributionhave the same change trend. And when at the same wear level, the4mm diameter350Ngroup has the highest stress value (High stress group);10mm diameter350N groupfollowed (Middle stress group);10mm diameter150N group has the lowest (Low stressgroup).2. The TCML test showing: During the2.4million loading cycles, the veneeringporcelain of all the three groups demonstrate tribological characteristics of time. Highstress group has shorter period of running in stage and steady wear stage(0-500,000cycles), while the sever wear stage sustains much longer(500,000-2,400,000cycles);Middle stress group has more evenly distributed of three wear stages(running in stage:0-500,000cycles, steady wear stage:500,000-1,500,000cycles, sever wear stage:1,500,000-2,400,000cycles); Low stress group only has running in stage(0-600,000cycles)and steady wear stage(600,000-2,400,000cycles).3. There are close relationship between microstructure, surface roughness and threecharacteristics wear stages. In running in stage, the surface of the material has intensivewear track, high value of surface roughness; In steady wear stage, some of the wear track become more sever, while some of them become more smooth. At the same time, cracksnucleate in the subsurface of the ceramic, and the surface roughness decreases. Whenreached the sever wear stage, subsurface crack propagate to the surface leading materialloss, and the surface roughness increases. All experiments show that the change tendencyof veneering porcelain material’s microstructure and physical parameters between thedifferent stress groups were similar. The high stress group has the biggest crack size, weararea, and lowest fracture load among three groups; low stress group has the smallest cracksize, wear area, and highest fracture load compared to other two groups; middle stressgroup centered.CONCLUSION:In a simulated oral environment, veneering porcelain demonstrate the tribologicalcharacteristics of time, which contain three characteristics of wear stages:"running-instage","stable wear stage" and "severe wear stage". The evolution of surface andsubsurface microstructure of veneering material and the changes of roughness arecorresponding to wear volume and wear rate in the certain wear stage. Different stressconditions have a certain influence on the tribological characteristics of time. The resultsprovide a comprehensive understanding of the wear process which contains macro-lawand micro-mechanism. It also have greater positive effect on designing ceramic prosthesisproperly, and can further make contribute to manufacture a new kind of dental ceramicwith outstanding performance.