Relationship between viscoelasticity and scratch morphology of coating films

In order to make clear the relationship between viscoelasticity of coating film and its scratch resistance, a number of clear coatings were examined by surface observation and measurements of their mechanical properties.

The scratches can be classified into two types by means of optical microscopic observation with attached polarizing filter, scraped fracture-type and plastic deformation-type. The ratios of the damaged areas to the whole observed areas were determined by image analysis as a measure of scratch resistance.

The comparison between the relaxation spectrum of the coatings and the ratio of the scratched area showed that the degree of surface damage caused by a scratch tester which simulated car washing machine correlated with the storage modulus of the coatings G_sr, at a relaxation time of 1 s. Moreover, the degree of plastic deformation by scratches depended also on S_pls, the minimum strain which could cause plastic deformation in the film.     

Investigation of AlN ceramic anisotropic deformation behavior during scratching

To explore the anisotropic deformation behavior of aluminum nitride ceramic during processing, ramp and constant load scratch experiments were conducted with Vickers indenter. Characteristics of deformation anisotropy were observed in ramp-load scratch with differences found in the directions of pile-ups, slip lines and cracks produced by grains with various orientations. Constant-load scratches were conducted to explore the anisotropic deformation behavior in plastic stage. Obvious differences were found in scratch pile-up height and residual depth of grains with different orientations. As the grain orientation Euler angle changes from the basal to the two prism surfaces, the residual depth of the scratch gradually increases. Molecular dynamics simulation was performed to disclose the mechanisms of anisotropic dislocation and deformation during scratching. The activation of the slip system for different grain orientations was calculated by a scratch Schmid factor model which was verified by the observation of pile-ups and slip lines around the scratches.     

A new approach to investigate scratch morphology and appearance of an automotive coating containing nano-SiO2 and polysiloxane additives

Attempts have been carried out to verify whether gonio-spectrophotometry could be utilized as a new scratch testing approach to investigate scratch morphology and its effect on appearance of an acrylic-melamine clearcoat separately containing nano-silica or polysiloxane additives. The results of gonio-spectrophotometry/colorimetry illustrated that this new approach is capable of differentiating between plastic and fracture types scratches. Furthermore, this approach was found to be suitable for analyzing the self healing abilities of such coatings. The accuracy and reproducibility of such results were checked against the results of scanning electron microscope (SEM) and illustrated great potential as a new approach for such studies.     

Surface and subsurface formation mechanism of SiCp/Al composites under ultrasonic scratching

Rotary ultrasonic machining (RUM) is an effective method of high-quality and high-efficiency machining for advanced composites. However, the machining mechanism and kinematic characteristics of ultrasonic machining of SiC particles-reinforced aluminum matrix (SiC_p/Al) composites are yet unclear, limiting the applications of RUM in composites machining. In this study, a rotary ultrasonic vibration-assisted scratch (RUVAS) test was designed for the high-volume fraction of SiC_p/Al composites. The kinematic and scratch force model of RUVAS was developed to describe the scratch process of SiC_p/Al. Both RUVAS and conventional scratch (CS) tests were performed under various scratch speeds on SiC_p/Al. The scratch trajectory was divided into three modes: continuous, semi-continuous, and intermittent. We observed the formation of different surface morphology under different modes. The scratch force difference between RUVAS and CS was insignificant when the scratch speed is high, which indicated that the effect of ultrasonic vibration diminished at a high speed when the ultrasonic frequency was fixed. When assisted by ultrasonic vibration, the scratch morphology of SiC_p/Al indicated that the matrix has undergone significant plastic deformation. While the hard SiC particles tended to be ruptured and pressed into the plastic matrix, this mechanism can effectively suppress the initiation and propagation of cracks, which is beneficial to reducing the stress influence zone, healing the surface defects, and improving the surface integrity. The subsurface morphology indicates that the subsurface damage under CS and RUVAS mainly includes particle cracking, matrix tearing, and interface failure. Our experimental result shows that ultrasonic vibration can effectively reduce the subsurface damage of SiC_p/Al composites, bringing insight into fundamental mechanisms of ultrasonic machining and providing guidance for the vibration-assisted processing of SiC_p/Al composites.     

Effect of Grain Size on Scratch Interactions and Material Removal in Alumina

Dramatic effects of scratch interactions on material removal are observed in alumina. A series of parallel scratches are made in aluminas with different grain sizes to investigate the influence of scratch interactions on the material removal process in abrasive machining. The separation distance between the two scratches and the normal load are varied and subsurface microfracture and damage modes are examined to assess the mechanisms of material removal. A very small amount of material is removed when the separation distance between the two parallel scratches is large or when the two scratches completely overlap. However, at intermediate distances the volume of material removed increases dramatically as a result of the interactions between the two scratches. The maximum amount of material removed and the corresponding distance between the two scratches are found to depend strongly on the grain size and the load. Observations of surface and subsurface damage reveal that grain dislodgement is the predominant mechanism of material removal, irrespective of the grain size. The relation between grain size, scratch interactions, and the material removal process in grinding and abrasive machining of ceramics is discussed in terms of the short-crack toughness of ceramics.     

Mechanisms of fatigue damage and fracture in semi-crystalline polymers

Fatigue crack profiles and fracture surfaces of poly(vinylidene fluoride) (PVDF), nylon-6,6 (N66), and poly(acetal) (PA) were studied to ascertain the mechanisms of cyclic damage and fatigue crack propagation in semicrystalline polymers. Crack tip damage is believed to begin as small trans-spherulitic and inter-spherulitic tensile crazes. However, compressive yielding within the reverse plastic zone at the crack tip crushes and elongates the spherulites in the direction of crack growth. Consequently, the microstructure of the polymer in advance of the crack front is different from the original morphology of the spherulitic bulk material as evidenced by the resulting fracture surface appearance. When the test temperature is below the glass transition temperature, however, plastic deformation is limited, and fatigue fracture occurs before significant disruption of the spherulitic structure. In this case, the fracture surface morphology reflects the original microstructure of the bulk polymer.     

Fracture mechanics of sharp scratch strength of polycrystalline alumina

The strength of a polycrystalline alumina containing controlled scratches introduced by translated sharp contacts is investigated and described by a multiscale fracture mechanics model. Inert strength measurements of samples containing quasi‐static and translated Vickers indentation contacts showed that scratches degraded the strength at normal contact loads an order of magnitude less than those for quasi‐static indentation. The fracture mechanics model developed to describe strength degradation by scratches over the full range of contact loads included toughening effects by crack‐wake bridging at the microscale and lateral crack‐based residual stress relaxation effects at the mesoscale. A critical element of the model is the nonlinear scaling of the residual stress field of a scratch with the normal contact load acting during scratch formation. The similarities and differences in the scratch model in comparison with prior indentation‐strength fracture mechanics models are highlighted by parallel development of both. Central to the scratch model is the use of easily controlled normal contact load as the scratch‐strength measurement variable. Scratch length and orientation are shown to have significant effects on strength. The distributions of scratch widths controlling the intrinsic strengths of as‐received samples are determined and agreement with the observed scratch dimensions is demonstrated.     

Sensitivity of Scratch Resistance to Grinding-Induced Damage Anisotropy in Silicon Nitride

Single-grit scratch experiments were conducted on polished as well as on longitudinally and transversely ground silicon nitride specimens to investigate the influence of grinding direction on scratch resistance. It was found that the volume of material removed and the extent of cracking were the largest when the scratches were parallel to the grinding direction and were the least on the polished specimens. These results were rationalized based on the grinding-induced sub-surface damage anisotropy and were quantified using a scratch resistance measure. A linear elastic fracture mechanics analysis revealed that only cracks of certain length and orientation grow to cause the observed trends in the scratch resistance.     

Ductile to Brittle Transition Depth During Single-Grit Scratching on Alumina Ceramics

Variable-depth single-grit scratch experiments have been conducted on three different grain size alumina ceramics. The extent of induced damage as a function of depth of groove was measured. At low depth, the scratch groove appeared smooth with minimal brittle damage, indicating a ductile mode of deformation. With increased depth, brittle cracking extended beyond the scratch groove. The transition depth from the predominantly ductile mode of deformation to the predominantly brittle mode was measured and compared with an analytical model that estimates the plastic zone size surrounding a scratch in brittle materials. It was found that the ductile to brittle transition depth increases with decreasing grain size.     

Study on the subsurface damage mechanism of optical quartz glass during single grain scratching

The single grain scratching SPH simulation model was established to study the subsurface damage of optical quartz glass. Based on the analysis of the stress, strain and scratching force during scratching, the generation and propagation of subsurface cracks were studied by combining with the scratch elastic stress field model. The simulation results show that the cracks generate firstly at the elastic-plastic deformation boundary in front of the grain (φ = 28°) due to the influence of the maximum principal tensile stress. During the scratching process, the median crack closes to form the subsurface damage by extending downward, the lateral crack promotes the brittle removal of the material by extending upward to the free surface, and microcracks remain in the elastic-plastic boundary at the bottom of the scratch after scratching. The depth of subsurface crack and plastic deformation increases with rising scratching depth. The increase of scratching speed leads to the greater dynamic fracture toughness, accompanied by a significant decrease of the maximum depth of subsurface crack and the number of subsurface cracks. The subsurface residual stress is concentrated at the bottom of the scratch, and the residual stress on both sides of the scratch surface would generate and propogate the Hertz crack. When the scratching depth is less than 1.5 μm or the scratching speed is greater than 75 m/s, the residual stress value and the depth of residual stress are relatively small. Finally, the scratching experiment was carried out. The simulation analysis is verified to be correct, as the generation and propagation of the cracks in the scratching experiment are consistent with the simulation analysis and the experimental scratching force indicates the same variation tendency with the simulation scratching force. The research results in this paper could help to restrain the subsurface damage in grinding process.     

Viscoelastic effects on the scratch resistance of polymers: relationship between mechanical properties and scratch properties at various temperatures

Scratch durability of polymer surfaces and coatings is becoming critical for the increasing use of these materials in new applications, replacing other materials with harder surfaces.

Scratch resistance of polymers has been the subject of numerous studies, which have led to specific definitions for plastic deformation characterization and fracture resistance during scratch testing. Viscoelastic and viscoplastic behavior during a scratch process have been related to dynamic mechanical properties that can be measured via dynamic nano-indentation testing. Yet, the understanding of the origin of the fracture process of a polymer during scratch remains approximate. Parameters like tip shape and size, scratch velocity and loading rate, applied strain and strain rates, have been considered critical parameters for the fracture process, but no correlation has been clearly established.

The goal of this work is to define and analyze scratch parameters that relate to mechanical properties. The evolution of scratch resistance parameters as a function of temperature and strain rate, compared to the evolution of dynamic mechanical properties obtained from indentation and uniaxial tensile tests over a range of temperature for poly(methyl methacrylate) (PMMA) helped in identifying a correlation between the tensile stress–strain behavior and scratch fracture toughness.

This correlation brings a new understanding of the origin of the fracture mechanisms during a scratch process. In particular, it is demonstrated that the characteristic strain applied by the indenter is a most relevant parameter to describe the fracture resistance during a scratch process, independently of the indenter geometry.     

Nanopatterning on silicon surface using atomic force microscopy with diamond-like carbon (DLC)-coated Si probe

Atomic force microscope (AFM) equipped with diamond-like carbon (DLC)-coated Si probe has been used for scratch nanolithography on Si surfaces. The effect of scratch direction, applied tip force, scratch speed, and number of scratches on the size of the scratched geometry has been investigated. The size of the groove differs with scratch direction, which increases with the applied tip force and number of scratches but decreases slightly with scratch speed. Complex nanostructures of arrays of parallel lines and square arrays are further fabricated uniformly and precisely on Si substrates at relatively high scratch speed. DLC-coated Si probe has the potential to be an alternative in AFM-based scratch nanofabrication on hard surfaces.     

Scratch resistant clear coats: development of new testing methods for improved coatings

More and more car manufacturers are now demanding clear coats with improved scratch resistance from their paint suppliers. Some motor companies have developed their own tests, some have chosen a test developed by a paint supplier and others are still looking for their optimum test. The opinion prevails that there should be one test which covers all aspects of realistic damage. However, microscopic photographs and reflow experiments show that two kinds of scratches occur in reality, abrasive as well as non-abrasive, renewable ones. Different standard clear coat systems have different sensitivities to both scratch types. In the scratch tests the rating of the clear coats is divergent, because the ratio of both sorts of scratches differs. We have developed a set of two tests, where each test mainly generates one scratch type. These tests, together with physico-chemical measurements (glass transition onset temperature T_g, laser optical profile scanning, indentation depths) lead to a deeper insight into scratch phenomena. One way to develop a clear coat which is resistant against all kinds of scratching damage is described.     

Studies of microhardness and mar resistance using a scanning probe microscope

Scanning probe microscopy (SPM) is in a period of rapid development. It shows great promise for characterizing coating surfaces. This paper describes modification of an SPM so that it can be used to mar the surfaces of coatings under controlled conditions and to characterize the mars. Mar resistance of coatings is analyzed in terms of a ‘three response, two mechanism model.' The three responses (fracture, elastic, and plastic) can be measured quantitatively using the SPM. Of the three responses, only two (fracture and plastic deformation) are marring mechanisms – elastic deformations recover instantaneously. In some cases mars resulting from plastic deformation may recover slowly with time or with immersion in water; this phenomenon is attributed to viscoelastic creep. Microhardness is also measured with the modified SPM. Some thermoset coatings appear to be substantially harder near their surfaces than in the mass of material, and such materials may respond quite differently to stress applied at different levels near the surface. This finding has important implications for all coating properties that are strongly influenced by the surface. A quantity called ‘micro mar resistance' is defined. It may be useful for comparing different coatings under specified conditions of marring. However, there can be no single quantity that expresses ‘mar resistance' of a coating under all conditions.     

Scratch Damage in Zirconia Ceramics

Scratch damage modes in zirconia-based ceramics—Mg-PSZ, Y-TZP, and Ce-TZP—are investigated. Precursor indentation tests with a tungsten carbide sphere foreshadow the nature of damage: in Mg-PSZ, extensive (quasi-)plastic deformation in the region outside and beneath the contact; in Y-TZP, less plastic deformation beneath the contact but incipient cone cracking in the region of tension outside the contact; in Ce-TZP, intermediate behavior. Scratch testing is conducted using a conical diamond indenter. In all materials the damage mode changes from smooth plastic deformation to limited cracking with increasing scratch load: in Mg-PSZ, plastic deformation is predominant at lower loads, with microcracking at higher loads; in Y-TZP, plastic deformation is predominant over the range of the test loads—macrocracks initiate only at relatively high loads, but penetrate to a relatively large depth; again, Ce-TZP shows intermediate behavior, but with cracking patterns closer to that of Mg-PSZ. Bending tests on specimens subjected to scratch damage indicate a relatively high damage tolerance in the Mg-PSZ and Ce-TZP; Y-TZP shows the highest initial strength, but suffers relatively large strength loss above the critical load for macrocracking. Implications concerning relative merits of each zirconia type for wear properties, contact fatigue, and machining damage are briefly discussed.     

Scratch behavior of boron nitride nanotube/boron nitride nanoplatelet hybrid reinforced ZrB2-SiC composites

Spherical instrumented scratch behavior of ZrB₂-SiC composites with and without hybrid boron nitride nanotubes (BNNTs) and boron nitride nanoplatelets (BNNPs) was investigated in this research. Typical brittle fracture such as microcracks both in and beyond the residual groove and grain dislodgement was observed in ZrB₂-SiC composite, while hybrid BN nanofiller reinforced ZrB₂-SiC composite exhibited predominantly ductile deformation. The peculiar three-dimensional hybrid structure in which BNNPs retain their high specific surface area and de-bundled BNNTs extend as tentacles contributes to the improved tolerance to brittle damage. Additionally, easier grain sliding due to BN hybrid nanofillers located at grain boundaries and these BN hybrid nanofillers attached on the scratch surface would provide significant self-lubricating effect to reduce lateral force during scratch and to alleviate contact damage.     

Use of a scanning probe microscope to measure marring mechanisms and microhardness of crosslinked coatings

A scanning probe microscope (SPM) was equipped with a high-modulus probe to indent coating surfaces when normal force is applied. A method for measuring microhardness with this probe is described. The high-modulus probe was also used to mar coating surfaces under controlled conditions by application of normal force plus lateral motion. Dimensions of the mars were measured by conventional scanning probe microscopy. The data were analyzed in terms of a “three response, two mechanism model” of marring in which three types of responses of polymeric materials: elastic, plastic deformation, and fracture, are measured. Of the three responses, only plastic deformation and fracture result in marring, and the two mechanisms can be quantified. A fourth quantity which combines plastic deformation and fracture is suggested as a method of comparing “micro mar resistance” of materials under specified conditions. Three crosslinked polymeric coatings were studied in detail. Two had hard crusts of material near their surfaces that responded quite differently than the bulk of the material. 430 W. Forest Ave., Ypsilanti, MI 48197. 401 Southfield Rd., Dearborn, MI 48121-6231.     

The Effect of Damage Nucleation on the Toughness of an Adhesive Joint

The intrinsic toughness of an adhesive joint has been shown to be different depending on whether the adherends remain elastic or deform in a plastic fashion. This phenomenon occurs because the different constraint imposed by the adherends results in a change in the deformation mechanisms of the adhesive layer. In the elastic geometry, damage nucleation occurs when the stresses in the adhesive layer reach a critical value before the conditions for fracture are met. Void growth then leads to large-scale bridging across the adhesive layer and an increase in the measured toughness. In contrast to this behavior, the reduced constraint associated with adherends that are thin enough to deform plastically allows the fracture criterion to be met before damage nucleation occurs. There is then no bridging contribution to the toughness. The effect of damage in an adhesive layer can be viewed either as a bridging zone behind the crack tip, or as an extended cohesive zone ahead of the crack tip. The toughness of an adhesive joint can either be increased or decreased by the nucleation of damage. The effects of a damage zone on the behavior of an adhesive joint with elastic adherends are discussed, and it is shown how numerical techniques can be used to model this behavior and to deduce the fracture parameters.     

The Effect of Damage Nucleation on the Toughness of an Adhesive Joint

The intrinsic toughness of an adhesive joint has been shown to be different depending on whether the adherends remain elastic or deform in a plastic fashion. This phenomenon occurs because the different constraint imposed by the adherends results in a change in the deformation mechanisms of the adhesive layer. In the elastic geometry, damage nucleation occurs when the stresses in the adhesive layer reach a critical value before the conditions for fracture are met. Void growth then leads to large-scale bridging across the adhesive layer and an increase in the measured toughness. In contrast to this behavior, the reduced constraint associated with adherends that are thin enough to deform plastically allows the fracture criterion to be met before damage nucleation occurs. There is then no bridging contribution to the toughness. The effect of damage in an adhesive layer can be viewed either as a bridging zone behind the crack tip, or as an extended cohesive zone ahead of the crack tip. The toughness of an adhesive joint can either be increased or decreased by the nucleation of damage. The effects of a damage zone on the behavior of an adhesive joint with elastic adherends are discussed, and it is shown how numerical techniques can be used to model this behavior and to deduce the fracture parameters.     

Effect of humidity on friction,wear, and plastic deformation during nanoscratch of soda lime silica glass

Physical flaws and defects on glass surfaces are known to reduce the mechanical strength and chemical durability of glass. The formation of surface defects depends not only on the mechanical conditions of the physical contact but also on the environment in which the contact is made. In this study, the nanoscratch behavior of soda lime silica (SLS) glass was investigated in 10% and 60% relative humidity (RH) conditions. Based on the evolution of friction and scratch depth, the deformation of SLS glass surface could be divided into four regimes: elastic deformation and recovery (E), RH-independent mild plastic deformation (P-1), RH-dependent intermediate plastic deformation (P-2), and RH-independent severe plastic formation (P-3). It is quite surprising to observe that plastic deformation of the glass surface has dependence on RH of the environment (outside the glass) because plastic deformation is the process occurring below the surface (inside the glass) by the externally applied load. From this result, it can be inferred that frictional energy dissipation mode at the sliding interface, which is a function of adsorbed water molecules, influences the subsurface deformation mode. Although friction, wear, and subsurface deformation/damage are all coupled, there is no direct one-on-one correlation among them.