Study On The Scratch Behaviors And Related Mechanism Of Polymethyl Methacrylate Based Multi-layer Assembled Materials

Scratch is defined as damage made by a hard asperity when moving across material’s surface,which mainly has negative effects on material’s surface quality.Due to the inherent mechanical characteristics of polymeric materials,they are susceptible to damage made by scratching.Therefore,polymeric materials with better scratch resistance are always in demand.Due to the outstanding scratch resistance of polymethyl methacrylate（PMMA）among thermoplastics,it has been widely used as automotive upholstery and cellphone backboard where high surface quality are required.One one hand,PMMA is a quite brittle glassy polymer,therefore it is usually used as laminate in practice application,in which ductile polocarbonate（PC）serves as a substrate layer.One the other hand,PMMA are still susceptible to damage made by scratching when in contact with metal products such as keys.Therefore,it is necessary to further enhance the scratch resistance of PMMA to better meet market requirement.In this paper,PMMA was used as a matrix.By taken its actual service condition into consideration,three typical polymers which possessed distinct scratch behaviors,namely,polyvinylidene fluoride（PVDF）,PC,UV-cured polyacrylate（HC）,were employed to construct PMMA based layered materials with different layer thickness,mechanical mismatch,interfacial bonding strength via a self-developed layer-multiplying co-extrusion instrument.The scratch behaviors of the prepared PMMA based layered materials were systematically studied in depth by both linear increasing normal load scratch test base on ASTM/ISO standards and finite element modeling（FEM）,which further revealed mechanisms during scratch deformation of polymers and contributed to a more comprehensive understanding on their scratch behaviors,and provided useful guidelines and new strategies to fabrication and developing of polymeric materials with better scratch resistance.The main research contents and results are as follows:1.The deformation and damage of layered polymers during scratching is as highly complicated nonlinear process.Introducing FEM into study on scratch behaviors of layered polymers and combining it with experimental analysis is the foundation to realize in-depth understandings on scratch behaviors of layered polymer.Therefore,based on the mechanical characteristics of polymers,finite element models for scratching of PMMA,PC and PVDF were firstly built by taken the high strain rate and high contact pressure features during scratch deformation of polymers into consideration.It was found that coefficient of friction during scratch modeling can be regarded as a constant value according to the variation tendency of contact pressure with normal load.Meanwhile,the strain rate sensitivity and dependence of polymers must be taken into account to realize more quantitative scratch modeling,i.e.,the change of compressive yield stress and post-yield behaviors must be included when determining constitutive behaviors.It was found that the post-yield behavior of PC and PVDF changed slightly at high stain rate,therefore quantitative scratch modeling of PC and PVDF were realized simply by considering their strain rate dependence.For PMMA,noteworthy heat softening occurred after yield during high speed compression.However,if its constitutive behaviors during scratch modeling was directly described by results obtained through high speed compression,then the simulated scratch size of PMMA was quite larger than its actual value.Then differences of heat transfer conditions during high speed compression and scratching was further analyzed by FEM.It was found that temperature rise during scratching was much lower than that during high speed compression.Therefore,the heat softening after yield during scratching was was much less prominent than that during high speed compression.The method for obtaining compressive constitutive parameters to realize more quantitative scratch modeling of PMMA was also proposed.It was suggested that the thickness of PMMA samples should be between 0.06 mm to 0.10 mm for a more accurate scratch modeling of PMMA.2.Base on amorphous glass PMMA and semi-crystalline ductile PVDF,which are highly compatible but possess distinct mechanical characteristics and scratch behaviors,alternating multi-layered PVDF/PMMA materials with the maximum layer number of 128 were fabricated by a self-developed layer-multiplying co-extrusion instrument.The scratch behaviors of PVDF,PMMA and alternating multi-layered PVDF/PMMA materials were studies,and the possibility of enhancing scratch resistance of PVDF and PMMA via layered structure was explored.Combining with FEM,the damage mechanisms of PVDF and PMMA during scratching were analyzed and their scratch damage criterions were established,and the mechanisms relating scratch behaviors of alternating multi-layered PVDF/PMMA materials were studied in-depth accordingly.FEM revealed that fish scale patterns formed during scratching of PVDF was attributed to the leaning of material piled up in front of scratch tip with tangential movement of scratch tip,follow by compressing of material at the same position when scratch tip moved over it.The scratch damage of PVDF was caused by tensile tearing at the two sides of material piled up in front of scratch tip,while that of PMMA was ascribed to compressive cracking below scratch tip due to its excessive compressive plastic deformation.When scratch tests were conducted on PVDF side of alternating multi-layered PVDF/PMMA materials,the material with a layer number of 32showed the highest critical normal load of scratch damage(F_cr),which was 40%higher than that of pure PVDF.This was a result of smaller tensile deformation of surface PVDF layer which was suppressed by subsurface PMMA layer.When scratch tests were conducted on PMMA side of alternating multi-layered PVDF/PMMA materials,the material with a layer number of 8 showed the highest critical normal load of scratch damage(F_cr),which was 25%higher than that of pure PMMA.A more uniform deformation of surface PMMA layer caused by plastic deformation of subsurface PVDF layer was found to be the cause of this phenomenon,which prevent excessive plastic deformation from concentrating on the contact surface of PMMA layer.Meanwhile,it was observed that the crack pattern on PMMA side of alternating multi-layered PVDF/PMMA materials was significantly different from that of pure PMMA,which matched the characteristics of tension-induced crack.FEM proved that this kind of crack was induced by the maximum principle stress behind scratch tip.The calculated critical normal load of tension-induced crack of pure PMMA was slightly higher than its critical normal load of material removal.Therefore,material removal preceded tension-induced crack in pure PMMA and led to absence of the latter.3.Based on the actual service condition of PMMA based laminate,partially compatible PMMA and PC were employed and alternating multi-layered PC/PMMA materials with the maximum layer number of 256 were fabricated.The scratch behaviors and damage mechanisms of PC and alternating multi-layered PC/PMMA materials were studied,and the effects of changing constitutive behaviors of soft layer component and interfacial bonding strength on scratch behaviors of PMMA based laminated were discussed.It was found that the maximum principle stress behind scratch tip induced scratch damage of PC.Compared with the previous PVDF/PMMA systems,the variation of F_crwith changing layer number were both small on two sides of PC/PMMA systems,which is closed related to the constitutive behaviors of soft layer.Meanwhile,prominent interfacial delamination occurred after onset of scratch damage of both sides of alternating multi-layered PC/PMMA materials with layer number of 16 and above.Experimental results demonstrated that disturbance induced by material removal rather than plastic deformation during scratching caused interfacial delamination between PC and PMMA.Then the detailed delamination process during scratching and related mechanisms was revealed by FEM.It was revealed that delamination process during scratching was dominated by shearing crack（Mode II&Mode III）propagation,whereas Mode I crack was hardly involved in this process.The interfacial delamination process during scratching can be divided into three successive stages.Stage I was delamination initiation,which was induced at center of scratch.Stage II was delamination expansion,in which the delamination zone fanned out symmetrically and rapidly towards two edges of scratch as normal load increased.Eventually the delamination process involved into stage III,delamination propagation,during which the delamination zone developed stably along scratch direction without width increasing.The interfacial stress in alternating multi-layered PC/PMMA materials did not monotonically decreasing with increasing depth,but was strongly related to the stacking sequence of material above and below the interfaces.Soft on hard interfaces were more likely to delaminate during scratching.The mechanical test results demonstrated prominent enhancements of the overall ductility of alternating multi-layered PMMA/PC materials with increasing layer number.Compared with 2L material,the elongation at break of 256L material increased by 243%,and the unnotched impact strength of 256L material increased by 447%.In addition,2L material broke at merely 3.8%flexural strain during bending test,while 256L material could endure doubling-over test without breaking.4.Alternating multi-layered PC/PMMA materials which were prepared in last chapter were employed as substrates,and hard coatings（HC）with average thickness of 5μm,10μm and 20μm were coated on both sides of substrates.The effect of surface HC on scratch behaviors of alternating multi-layered PC/PMMA materials were studied by both spherical tip scratch test with linear increasing normal load and pencil hardness test with constant normal load.Nano indentation results demonstrated that the hardness of HC was 1.64 times larger than PMMA,and 2.42times larger than PC.Results of spherical tip scratch test showed that all HC-PC/PMMA systems demonstrated similar damage features.As normal load increased,smooth groove appeared first,followed by（（（type cracks,then）））type cracks formed.Interaction between（（（type cracks and）））type cracks led to breaking up of HC,and ultimately material removal of the underlying alternating multi-layered PC/PMMA substrates occurred.（（（type cracks formed an angle of 45°with scratch direction.FEM revealed that the maximum principle stress which also formed an angle of 45°with scratch direction and located at two rear side of scratch tip induced（（（type cracks.It was observed that）））type cracks was formed at the peak of material piled up in front of scratch tip,and it was deduced that the formation of）））type cracks was attributed to bending-induced fracture of HC at the same area caused by excessive bending deflection as the height of material piled to a certain value.The critical normal load of（（（type cracks and）））type cracks were both found to be closely related to hardness of substrates,which increased with increasing hardness of substrates.The visibility of scratches formed after pencil test can be attribute to plastic deformation of the underlying substrate,or the periodic cracks formed in HC,or the combination of both.During pencil hardness test,periodic cracks was initiated by the maximum principle stress which was parallel to scratch direction and located at the interface of HC and substrate beneath the contact zone between pencil tip and HC.Pencil hardness of HC systems were also found to be closely related to hardness of substrates,which also increased with increasing hardness of substrates.