Scratch Coefficient Of Friction Research Articles

Scratch-induced deformation and damage behavior of doped diamond-like carbon films under progressive normal load of Vickers indenter

Microscratch test was conducted on four different diamond-like carbon (DLC) films by Vickers indenter to study the scratch resistance of films under progressive normal load linearly increasing from 5 mN to 15 N. With the increase of normal load, penetration depth, lateral force, residual and contact scratch widths increase nonlinearly; scratch hardness and lateral hardness decrease nonlinearly; scratch friction coefficient increases under small loads, and then tends to be a constant under large loads. DLC-H has the smallest scratch friction coefficient among the four films, and different targets have little effect on scratch friction coefficient. DLC-Si and DLC-Cr have almost the same scratch resistance (or penetration depth), and larger than that of DLC, whose scratch resistance is almost the same as that of DLC-H. Fracture toughness was obtained by scratch-based methodologies: the additional Si or Cr target can enhance fracture properties of DLC film-steel substrate systems, whereas C4H10 can harm fracture properties. Fracture toughness of DLC-Si (about 17 MPa·m1/2) was larger than that of DLC-Cr (about 15 MPa·m1/2) with the large values of fracture toughness obtained under penetration depth larger than film thickness attributed to substrate effect.

Quantitative modeling of scratch behavior of amorphous polymers at elevated temperatures

The scratch resistance of polymeric materials is generally known to deteriorate with increasing temperature of testing. The present study focuses on quantitatively predicting the effect of temperature on the scratch behavior of amorphous polymers. The Arruda-Boyce viscoplastic model is utilized to account for temperature and strain rate dependent strain-softening and strain-hardening behaviors. The post-yield behavior predicted in this model is calibrated using the yield point determined by the Richeton cooperative model. The pressure dependent Drucker-Prager model with calibrated post-yield experimental data at various strain rates is chosen as the plastic constitutive relationship of the polymeric systems for FEM simulation. Furthermore, temperature and pressure dependent frictional behavior is input into an ABAQUS contact model to simulate the variation of the adhesion coefficient of friction (μa) during the ASTM standardized linearly increasing load scratch test. The FEM simulation findings show a good agreement with the experimentally determined scratch depth and scratch coefficient of friction (SCOF) measured using the scratch test. Usefulness of the present study for design of scratch resistant polymers at elevated temperatures is discussed.

Experimental and numerical determination of adhesive strength in semi-rigid multi-layer polymeric systems

A test methodology to evaluate the adhesive strength of poly(ethylene terephthalate) (PET) multi-layer articles was developed by implementing a linearly increasing normal load scratch test (ASTM D7027-13/ISO 19252:08). The adhesive failure load between the layers and the failure modes near or at the interface were experimentally determined. The load required to induce adhesive failure and the scratch coefficient of friction were determined from the scratch test. The role of substrate deformability on adhesive failure during the scratch test was also investigated. Finite element method (FEM) modeling was carried out to investigate the corresponding stress state that causes adhesive failure to occur during scratching. FEM simulations allow for the geometric factors and material constitutive behavior to be taken into account in determining the interfacial stresses in the multi-layer system. By combining the scratch test with FEM modeling, the proposed approach can semi-quantitatively assess the delamination strength of semi-rigid multi-layer polymeric systems.

Load effect on scratch micro-mechanisms of solution strengthened Compacted Graphite Irons

Abstract This study investigates the scratch load effect, from 100 to 2000 mN, on micro-mechanisms involved during scratching. A pearlitic and three ferritic Compacted Graphite Irons (CGI) solution strengthened through addition of 3.66, 4.09, and 4.59 Si wt% were investigated. Good correlation was observed between hardness measurements, tensile testing, and scratch results explaining the influence of matrix characteristics on scratch behaviour for investigated alloys. A significant matrix deformation, change in frictional force and scratch coefficient of friction was observed by increase in scratch load. In all cases, microscratch depth and width increased significantly with load increasing, however pearlitic CGI showed most profound deformation, while the maximum and minimum scratch resistances were observed for high-Si ferritic and pearlitic CGI alloys, respectively.

Analytical model of friction behavior during polymer scratching with conical tip

To investigate the effects of the contact geometry, interfacial friction, and substrate recovery on the behavior of polymer scratching using a conical tip, an analytical model is proposed. The normal stress acting on the contact surface between the tip and the substrate is described as a function of the included angle θ, representing the angle between two planes across the axis of the conical tip, and the attack angle β, representing the angle between the conical surface and the substrate material surface. The effects of the rear contact geometry on the scratch friction between the tip and substrate, represented by recovery angle φ, owing to the instantaneous elastic recovery of the polymer substrate, are also introduced. Validated by the experimental and numerical results from the literature, the proposed analytical model can describe well the scratch coefficient of friction (SCOF), which is defined as the ratio of the tangential force to the normal force. Meaningful guidance is provided to understand the scratch friction behavior.

Effect of Blend Composition on Scratch Behavior of Polystyrene/Poly(2,6‐dimethyl‐1,4‐phenyleneoxide) Blends

AbstractThe relationship between the scratch behavior and molecular aggregation states of polystyrene (PS), poly(2,6‐dimethyl‐1,4‐phenyleneoxide) (PPO), and their blends, is investigated based on differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD), polarized optical microscopy (POM), and indentation and scratch tests. DSC reveals that all the PS/PPO blends show the single glass transition temperature (Tg) and the Tg monotonically increase and Tg breadth exhibits a maximum, with an increase in PPO content. Furthermore, density and intermolecular chain distance obtained by WAXD exhibits maximum and minimum values at near 50 wt% of PPO, respectively. It is evident that densification occurs by blending PS and PPO. The scratch coefficient of friction (SCOF) value of PS is the largest and PS exhibits a fish‐scale pattern after scratch testing, while the SCOF value of PPO is much smaller than PS and PPO exhibits smooth groove formation. The PS50/PPO50 and PS20/PPO80 blends exhibit superior scratch and indentation resistance than PS and PPO. Damage morphology observation by POM and indentation tests reveals that molecular orientation is more restricted, and resistance against indentation increases for blends. This is due mainly to densification of the blend system.

Tribology behavior on scratch tests: Effects of yield strength

A three-dimensional (3D) scratch model is proposed to investigate the effects of yield strength of both coatings and substrates. With the help of combined Coulomb and plastic friction, the obtained results comprehensively interpret the experimental phenomena in most metals that with the growth of hardness after heat treatment the scratch friction coefficient (SFC) increases. This interpretation could not be done before. Scratch tests on the surface with or without the coating are discussed. Without the coating the SFC increases due to the decrease of the area with plastic slippage and/or the increase of friction stress during the increase of the yield strength in the material. With a softer substrate the friction stress decreases but the SFC increases, which is caused by the growth of the entire contact area and surface deformation. Conversely, with a stronger substrate the SFC decreases due to an intensified plastic slippage. The obtained results pave a new way to understanding the effects of yield strength on scratch tests, interpret experimental phenomena, and should be helpful for an optimum design in experiments.

Effects of surface roughness on scratch resistance and stress-strain fields during scratch tests

A three-dimensional scratch model is proposed to numerically investigate the effects of surface roughness on the scratch resistance, stress concentration, residual stress and plastic deformation during scratch tests, with the help of the finite element method. Without loss of generality, the surface roughness of a coating is modelled by a sinusoidal function. The stress and plastic strain fields with various geometry parameters of roughness are obtained and discussed. In comparison with a smooth coating, the stress concentration, the residual stress, and the scratch resistance are significantly intensified and the plastic strain is increased by a factor of five in a corrugated coating, which may tremendously reduce the material performance. Consequently, the effects of surface roughness should not be ignored in both experiments and simulations. The geometry of roughness is determined by both the wave amplitude and wavelength, and their influences on the tribological behaviors can be significant. While existing experiments are only focused on the roughness in vertical direction (corresponding to the wave amplitude), the effects of wave amplitude and wavelength can be qualitatively different in many aspects and should be considered separately. The scratch depth and the area of contact region between the indenter and coating are not very sensitive to a moderate change in the wave amplitude, while they are reduced obviously with a rising wavelength. While the stress concentration increases monotonously with a rising amplitude, it becomes more complex and the decrease of wavelength can reduce the stress concentration in some cases. A growth in roughness by a large wave amplitude or a small wavelength leads to an enhancement on scratch friction coefficient and friction resistance, which is qualitatively consistent with experiments. The findings here contribute to a new way for quantitative evaluations of the effect of surface roughness and should be beneficial to modelling and simulation in scratch tests and also to the optimum designs on the surface coating for experiments.

Enhancing Mar and Abrasion Resistance of Acrylic Hard Coatings with Soft Base Layer

Mar and abrasion resistance were investigated by a progressive load scratch test and steel wool abrasion test, respectively. Two acrylic coating systems including trimethylolpropane triacrylate (TMPTA) and pentaerythritol triacrylate (PETA) were prepared. A soft base layer was introduced as an intermediate layer between two different types of top layer and poly (methyl methacrylate) (PMMA) substrate to demonstrate the effect of soft base layer on mar and abrasion resistance. Abrasion damage on the coating surface was found to be less severe, when the soft base layer was incorporated into the coating systems. The reduction in scratch coefficient of friction (SCOF) and surface roughness was also observed. The results suggested that mar and abrasion resistance was greatly influenced by the presence of soft base layer, although different top layers were used. Moreover, it was found that abrasion resistance was further improved as the thicker soft base layer was applied.

Scratch resistance of high performance polymers

Scratch tests were carried out on various high performance polymers, including (1) polybenzimidazole (PBI), (2) polyparaphenylene (PPP), (3) polyetheretherketone (PEEK), and (4) polyimide (PI). The scratch damage features were characterized using laser confocal and scanning electron microscope. Scratch resistance at room temperature decreased in the same order as the materials are listed above. It was attempted to correlate the scratch depth with basic mechanical properties, such as Young's modulus, tensile strength, and scratch hardness. Also, the scratch coefficient of friction was considered as a possible measure to differentiate between the various materials tested.

Influence of surface roughness and contact load on friction coefficient and scratch behavior of thermoplastic olefins

To study the effects of surface roughness and contact load on the friction behavior and scratch resistance of polymers, a set of model thermoplastic olefins (TPO) systems with various surface roughness ( R a) levels were prepared and evaluated. It is found that a higher R a corresponds to a lower surface friction coefficient ( μ s). At each level of R a, μ s gets larger as contact load increases, with a greater increase in μ s as R a level increases. It is also observed that with increasing contact load and increasing R a, the μ s tend to level off. In evaluating TPO scratch resistance, a lower μ s would delay the onset of ductile drawing-induced fish-scale surface deformation feature, thereby raising the load required to cause scratch visibility. However, as the contact load is further increased, the μ s evolves to become scratch coefficient of friction (SCOF) as significant sub-surface deformation and tip penetration occur and material displacement begins, i.e., ploughing. No dependence of R a and μ s on the critical load for the onset of ploughing is observed. In this work, the distinction between μ s and SCOF will be illustrated. Approaches for improving scratch resistance of polymers via control of R a are also discussed.

Effects of molecular weight and thermal history on scratch behavior of polypropylene thin sheets

The effects of molecular weight ( M w) and thermal history on the scratch behavior of polypropylene (PP) thin sheets and a commercial thermoplastic olefin (TPO) have been investigated. Scratch parameters like the critical load for onset of scratch visibility and scratch coefficient of friction (SCOF) were utilized in this evaluation. The results suggest that scratch performance is improved when the M w and surface crystallinity of PP are high. Correlation between scratch resistance and surface crystallinity of PP is established and discussed. Approaches for the preparation of scratch-resistant TPOs are also addressed.