Lubricant Testing Research Articles

Effect of synovial fluid constituent on the tribological behaviors of UHMWPE-CoCrMo alloy contact pair

PurposeThis study aims to systematically investigate the tribological behaviors of metal and polyethylene using lubricants composed of four synovial fluid (SF) components. In addition, the changes in protein conformation during wear were analyzed to establish the correlation between protein conformation and tribological properties.Design/methodology/approachA pin-on-disk tester with multidirectional sliding motion was used for tribological properties observation between metal and polyethylene pairs. Simulated SFs with four main constituents were used as the testing lubricants. Differential scanning calorimetry and Raman were used to characterize the changes in protein conformation during wear.FindingsThe coupling of lipids and hyaluronic acid further suppressed protein denaturation. The protein structures of the adsorption film and the ensnared protein chains in the friction zone were maintained to a certain extent, thus improving the friction and wear of polyethylene.Originality/valueThese findings established the correlation between protein conformation and friction and wear, promoting the understanding of the lubrication mechanism of artificial joints.

Soot wear mechanisms in heavy-duty diesel engine contacts

In this study, the wear mechanisms of a used, soot-containing oil drained from an engine test are investigated. As reviewed at the start of this paper, many wear mechanisms have been proposed for soot-containing oils. Often these studies were conducted using fresh oils with added carbon black as a soot mimic which do not always accurately represent the oxidation pathways in engines. Hence, we apply a range of tribometer tests and surface analyses techniques to assess the wear performance of engine-drained oil on AISI 52100 steel. Results suggest that this used oil gives rise to the corrosive-abrasive wear mechanism involving repeated cycles of anti-wear film formation and subsequent removal by soot abrasion. The role of different additives in mitigating/exacerbating this mechanism are studied by testing lubricant samples with key additives omitted. This reveals that detergent helps to form a strong, soot-resistant tribofilm, while ZDDP prevents wear provided it can form a strong film but can also increase wear in a used, abrasive, soot-containing oil.

Review of the evolution and prevention of friction, wear, and noise for water-lubricated bearings used in ships

With the development of green tribology in the shipping industry, the application of water lubrication gradually replaces oil lubrication in stern bearings and thrust bearings. In terms of large-scale and high-speed ships, water-lubricated bearings with high performance are more strictly required. However, due to the lubricating medium, water-lubricated bearings have many problems such as friction, wear, vibration, noise, etc. This review focuses on the performance of marine water-lubricated bearings and their failure prevention mechanism. Furthermore, the research of marine water-lubricated bearings is reviewed by discussing its lubrication principle, test technology, friction and wear mechanism, and friction noise generation mechanism. The performance enhancement methods have been overviewed from structure optimization and material modification. Finally, the potential problems and the perspective of water-lubricated bearings are given in detail.

Testing of Lubricants for DIC Tests to Measure the Forming Limit Diagrams of Aluminum Thin Sheet Materials

We investigated lubricants and thin teflon foils that can be applied in the formability testing of the thin aluminum sheets used in the electronics and automotive industries. For the tests, thirteen different industrial lubricants (oils and greases) (i.e., L1–L13) and two Teflon films (i.e., 0.08 and 0.22 mm thick) were applied. The authors conducted an Erichsen test, and the thickness reduction of the discs was measured first. In forming-limit curve (FLC) tests, it is crucial that the stresses are localized in the central area of the specimen during forming and that biaxial deformation is maintained throughout, if possible. We aimed to achieve and fulfill this task. To perform this measurement, the GOM ARAMIS measuring system was utilized. It is an optical measuring system based on the digital image correlation (DIC) principle, capable of measuring both stresses and displacements in real time. A specific validation method was also developed to qualify the DIC system. We concluded that there was a 5% difference in Erichsen indentation (IE) number diagrams between the best (L12) and worst (L4) cases for the lubricants and oils tested, which is a significant difference for thin plates. We found that this value could be increased and improved by using Teflon discs. Furthermore, the localization of stress maxima, i.e., the centering of cracks in the specimen, could be achieved by combining Teflon discs and L12 lubricant (with appropriate layer order), which significantly aids in the recording of standard FLC diagrams. Using foils is also advantageous because they are readily available, have no expiry date, and are of less environmental concern.

Wear Performance of a Novel Silicon Nitride Ceramic for Biomedical Applications

Ceramic materials are sometimes used as bearing surfaces of joint implants due to their high wear resistance and biocompatibility. Silicon nitride (Si3N4) is one of the ceramics under investigation for such use, owing to its bacteriostatic properties and high wear resistance. Traditional Si3N4 is sintered using Al2O3 and Y2O3 as sintering aids. To improve the biocompatibility and bioactivity of Si3N4, new sintering aids (SrO, MgO, and SiO2) were used in this work. This substitution may however have substantial effects on the wear properties of the material. Hence, the aim of this study was to evaluate these effects. Multidirectional pin-on-disc wear tests against ultra-high molecular weight polyethylene pins were used to this end, running in fetal bovine serum solution at 37 °C for 2 million cycles. The surface roughness, phase composition, and surface morphology of the surfaces were investigated, together with the pH of the wear test lubricant and compared to a traditional Si3N4 composition. XRD and SEM results showed that the prepared ceramics were composed of α- and β-phase Si3N4. The surface roughness of the Si3N4 ceramic discs was in accordance with the biomedical standard, and the wear test results demonstrated that the new ceramics had a low wear factor and a comparable coefficient of friction to Si3N4 ceramics reported in previous work. However, the dissolution of the materials over time may be a concern for biotribological applications, such as long-term use or in other wear couples.

Wear monitoring method of water-lubricated polymer thrust bearing based on ultrasonic reflection coefficient amplitude spectrum

The water-lubricated thrust bearings of the marine rim-driven thruster (RDT) are usually composed of polymer composites, which are prone to serious wear under harsh working conditions. Ultrasonic is an excellent non-destructive monitoring technology, but polymer materials are characterized by viscoelasticity, heterogeneity, and large acoustic attenuation, making it challenging to extract ultrasonic echo signals. Therefore, this paper proposes a wear monitoring method based on the amplitude spectrum of the ultrasonic reflection coefficient. The effects of bearing parameters, objective function, and algorithm parameters on the identification results are simulated and analyzed. Taking the correlation coefficient and root mean square error as the matching parameters, the thickness, sound velocity, density, and attenuation factor of the bearing are inversed simultaneously by utilizing the differential evolution algorithm (DEA), and the wear measurement system is constructed. In order to verify the identification accuracy of this method, an accelerated wear test under heavy load was executed on a multi-functional vertical water lubrication test rig with poly-ether-ether-ketone (PEEK) fixed pad and stainless-steel thrust collar as the object. The thickness of pad was measured using the high-precision spiral micrometer and ultrasonic testing system, respectively. Ultimately, the results demonstrate that the thickness identification error of this method is approximately 1%, and in-situ monitoring ability will be realized in the future, which is of great significance to the life prediction of bearings.

Synthesis and characterization of size controlled nano copper oxide structures for antioxidant study and as eco-friendly lubricant additive for bio-oils

This research work aims at the synthesis of Copper oxide (CuO) nanostructures with the application as additives for biolubricant oils, particularly Pongamia oil. The CuO particles were synthesized by a simple co-precipitation methodology. The nanostructures were characterized via analytical and microscopic techniques. The p-XRD analysis reveals the formation of size-controlled CuO formation, and the particle size was affirmed using both scherrer and William son Hall methods with particle size less than 14 nm. The surface morphology studies were substantiated by microscopic techniques. Elemental analysis reveal the purity of the prepared nanostructures. The monodispersed nanostructures are obtained with polydispersity index (PDI) of a value of 0.0550. The antioxidant property of CuO NPs was analyzed and is found in dose dependent manner, with efficiency of 71.9% at 50 μg/mL The influence of CuO nanoparticles on the viscosity of the lubricant is studied using the rheometer. The effect of these nanostructures on the friction characteristic is studied using a Ring-On-Block lubricity tester. The particles varied from 0.5 to 1 % wt. The CuO nanoparticles reduces the viscosity of the pongamia oil bio lubricants by 5.52% in comparision with the pongamia oil blended neem oil. The coefficient of Friction (COF) was found to be superlative for 0.75 % wt CuO added to Pongamia oil. The experimental result shows that CuO nanostructures is more effective than the blending of Neem oil with Pongamia oil and cane be used as an additives in the bio lubricants as the viscosity and friction modifier.

Development of lubrication film and influence on friction in a total knee replacement during a gait cycle

The presented study dealt with the experimental analysis of friction and lubrication mechanism observed with fluorescent microscopy in total knee replacement represented by CoCrMo wheel and PMMA plate. Model synovial fluid with marked constituents was used as test lubricant. The experiments investigated three main parameters. Slide-roll-ratio (SRR) showed the greatest influence on lubrication film formation. Friction coefficient was mostly influenced by axial load; its increase led to a decrease in friction. Relative speed was manifested for SRR 1.25 and higher where its increase caused a decrease in friction. The complex experiment showed a development of friction coefficient and lubrication film along the whole gait cycle. The results of this experiment correspond with the initial results concerning the influences of parameters.

BaTiO3 modified 3Y-TZP with self-lubricating property for dental application

The present study aimed to evaluate the wear behavior of BaTiO3 (BT) modified 3Y-TZP ceramic composites compared to commercially available 3Y-TZP products and provided evidence relevant to its clinical use. A set of BT-modified 3Y-TZP (BT/3Y-TZP) ceramic composites was prepared. The mechanical properties and wear behavior of BT/3Y-TZP ceramic composites were investigated. The 3D digital microscopy, scanning electron microscopy (SEM), and scanning transmission electron microscopy (STEM) analyses of the worn surface helped to reveal the underlining mechanisms. In comparison with commercially available 3Y-TZP ceramics, the mechanical properties of the BT/3Y-TZP composite ceramics could meet the clinical requirements. More importantly, the 5 mol% BT/3Y-TZP showed the best wear properties against natural tooth enamel for the dry and saliva lubrication tests. The SEM and STEM analyses confirmed a stable transfer enamel layer formed on the surface of 5 mol% BT/3Y-TZP ceramic composite under dry as well as saliva environments. The in site formed tribofilm with self-lubricating function contributed to the reduced wear of the antagonist teeth. Therefore, the enamel-friendly 5 mol% BT/3Y-TZP may be a new material option for restorative dentistry. Moreover, the interface transfer theory could provide a new way of thinking to find more enamel-friendly dental restorative materials in the future.

Elongational behaviour of electrostatically stabilised and concentrated CNF and CNC hydrogels: Experiments and modelling

TEMPO-oxidized cellulose nanofibril (CNF) hydrogels or cellulose nanocrystal (CNC) hydrogels can now be obtained at high concentrations (>10 wt%) and used to fabricate biobased materials and structures. Thus, it is required to control and model their rheology in process-induced multiaxial flow conditions using 3D tensorial models. For that purpose, it is necessary to investigate their elongational rheology. Thus, concentrated TEMPO-oxidized CNF and CNC hydrogels were subjected to monotonic and cyclic lubricated compression tests. These tests revealed for the first time that the complex compression rheology of these two electrostatically stabilised hydrogels combines viscoelasticity and viscoplasticity. The effect of their nanofibre content and aspect ratio on their compression response was clearly emphasised and discussed. The ability of a non-linear elasto-viscoplastic model to reproduce the experiments was assessed. Even if some discrepancies were observed at low or high strain rates, the model was consistent with the experiments.

Digitally-enhanced lubricant evaluation scheme for hot stamping applications

Digitally-enhanced technologies are set to transform every aspect of manufacturing. Networks of sensors that compute at the edge (streamlining information flow from devices and providing real-time local data analysis), and emerging Cloud Finite Element Analysis technologies yield data at unprecedented scales, both in terms of volume and precision, providing information on complex processes and systems that had previously been impractical. Cloud Finite Element Analysis technologies enable proactive data collection in a supply chain of, for example the metal forming industry, throughout the life cycle of a product or process, which presents revolutionary opportunities for the development and evaluation of digitally-enhanced lubricants, which requires a coherent research agenda involving the merging of tribological knowledge, manufacturing and data science. In the present study, data obtained from a vast number of experimentally verified finite element simulation results is used for a metal forming process to develop a digitally-enhanced lubricant evaluation approach, by precisely representing the tribological boundary conditions at the workpiece/tooling interface, i.e., complex loading conditions of contact pressures, sliding speeds and temperatures. The presented approach combines the implementation of digital characteristics of the target forming process, data-guided lubricant testing and mechanism-based accurate theoretical modelling, enabling the development of data-centric lubricant limit diagrams and intuitive and quantitative evaluation of the lubricant performance.

Assessment of frictional torque and lubricity of waste-tire bio-oil as lubricant additive for stuck pipe

The optimal approach to reducing surface torque when drilling uses oil-based or synthetic-based mud systems instead of aqueous mud systems. However, lower surface torque and drag can be achieved by lowering the water-based mud friction factors using lubricant additives. Torque can be reduced up to 25% with the suitable mud lubricant additives. Lubricant additives are viable methods for decreasing frictional resistance in drill strings during oil and gas drilling operations. In this study, bio-oil was produced from waste tire distilled pyrolytic oil and added to formulated standard aqueous drilling fluid to investigate its ability to reduce frictional torque during drilling. The lubricating properties of the proposed lubricant additive was compared with commercially used lubricant additive as reference samples (control). The simulation results were compared with measurements from the OFITE Lubricity Tester. The result shows that adding the distilled waste tire pyrolytic oil and produced bio-oil to the drilling mud decreased the coefficient of friction and torque to generate oily films on the contact-surface. The distilled pyrolytic oil and produced bio-oil torque values were in the range of 23.50–32.98 and 21.30–30.82 respectively. The commercial lubricant additive (control) gave 42.85% maximum torque reduction, while the produced bio-oil gave a similar maximum torque reduction of 41.88%. The result shows that, for a water-based drilling fluid, the friction coefficient between the drill string and rock formation, can be reduced with the application of the produced bio-oil. Thus, the produced bio-oil can be used to prevent differential sticking or free the stuck pipe during drilling.

Effect of a stabilizer on the structure, synthesis, and tribological properties of silver nanoparticles

The structure and properties of nanoparticles are greatly influenced by the synthesis method and the stabilizer used in their preparation. The paper presents the results of the effect of various stabilizers (gelatin, sodium citrate, sodium borohydride) on the structure, synthesis, and tribotechnical properties of silver nanoparticles. The mechanism of the formation and growth of silver nanoclusters was investigated using quantum-chemical calculations with full optimization of all parameters using the density functional theory (DFT) PBEPBE/Lanl2DZ method. It is shown that the use of organic stabilizers (for example, gelatin) leads to an increase in the stability of stabilized silver clusters. Such systems have greater conformational flexibility and a large number of molecular donor oxygen centres in their composition. The effect of the stabilizer on the structure of the Ag13 silver cluster was revealed in four model systems: gelatin, ammonium chloride NH4Cl, sodium borohydride NaBH4, and trisodium citrate Na3C6H5O (Na3Cit). Silver nanoparticles (AgNPs) obtained by the sonoelectrochemical, citrate, borohydride, and citrate-borohydride method varied in size from nanometers to several tens of nanometers. As a result of tribotechnical tests of lubricants with the obtained silver nanoparticles, a positive effect of stabilized silver clusters was revealed in the composition of lubricant compositions. The best tribotechnical characteristics were shown by lubricating compositions containing AgNPs stabilized with gelatin. It was found that a decrease in the size of gelatin stabilized AgNPs, when added to lubricants, leads to an increase in the tribological properties.

Evolution of grease lubrication regimes and surface damage during reciprocation-oscillation transformation

The responses of two kinds of lithium greases during the transformation from reciprocating motion to oscillation were firstly studied on a ball-on-disk test rig using optical interferometric technique. Although in pure rolling motion, after a long run a thin film lubrication (TFL) state is formed on the track, in simple sliding motion an interior boundary lubrication (BL) region is quickly developed in the contact after only tens of working cycles and the region increases with the increase of the radius stroke ratio. Microstructure of the thickener fiber makes a difference in the formation of the inside BL region. By an SRV tester, grease lubricated wear tests were carried out using samples with smooth surfaces, textured surfaces and textured + diamond like carbon (DLC) coated surfaces in simple sliding reciprocating and oscillating motion. It is seen that lubrication using grease with thinner and evenly distributed thickener fibers produces less severe surface damage. The smooth or textured surface is soon worn out, causing severe surface damage. The application of DLC coating onto the textured surface can protect the surface well but increase the friction coefficient a little bit.

Synthesis and characterization of novel ethyl levulinate coupled N-phenyl-p-phenylenediamine multifunctional additive: oxidation stability and lubricity improver in biodiesel

ABSTRACT Biodiesel is an emerging solution to petrodiesel owing to its high flash point, low toxicity, biodegradability, and less greenhouse gas (GHG) emissions. It is still a problem for end-users due to poor oxidation stability and cold flow properties. However, biodiesel can be used as a lubricity improver in petrodiesel, but high dosages of biodiesel may be cost-intensive. Multifunctional additives can provide a robust solution to the said challenge. We report herein the Schiff base ethyl levulinate coupled with N-phenyl-p-phenylenediamine (EL-NPPD) as multifunctional additive, viz. antioxidant and lubricity enhancer for the biodiesel. The additive was synthesized in two steps: ethyl levulinate (EL) production followed by imine formation. The final product was characterized using thermogravimetric analysis (TGA), Fourier transform infrared (FTIR), and nuclear magnetic resonance (NMR) spectroscopy. The antioxidant properties of the synthesized additive were checked in terms of induction time using Rancimat as per EN14112 methods. At the 1000 ppm concentration, the induction period value of 7.94 ± 0.6 was observed. Further, as the concentration increased, induction time got increased. Furthermore, the antioxidant potential was checked with 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical inhibition-based method in which different concentrations of additive ranging from 1000 to 5000 ppm were used against commercial antioxidants. As concentration increased to 4000 ppm, % inhibition values got increased by 15%, which showed better antioxidant activity of EL-NPPD. Similarly, the lubricity tests were performed using the HFRR test in which additive ranging from 1000 to 2000 ppm was used, and the average wear scar diameter (AWSD) was significantly reduced from 258 ± 11 to 248 ± 10, i.e. around 5% reduction than neat biodiesel sample.

Lubrication Performance of Sunflower Oil Reinforced with Halloysite Clay Nanotubes (HNT) as Lubricant Additives

This study evaluates the tribological performance of nanolubricants of a vegetable oil (sunflower oil) reinforced with different concentrations of environmentally-friendly nanoparticles of halloysite clay nanotubes (HNTs). Tribological characterization was performed under different conditions to determine its effect on the nanolubricants’ performance and optimal HNT concentration. The tribological performances under low and high contact pressures were analyzed with a block-on-ring tribometer following the ASTM G-077-05 standard procedure. The extreme pressure (EP) properties of the nanolubricants were determined with a T-02 four-ball tribotester according to the ITeE-PIB Polish method for testing lubricants under scuffing conditions. In addition, the lubrication performance of the newly-developed vegetable oil-based nanolubricants was evaluated in an industrial-type application through a tapping torque test. The results indicated that at a low contact pressure 1.5 wt.% HNTs/sunflower oil provided the best tribological behavior by decreasing the coefficient of friction (COF) and wear volume loss by 29 and 70%, respectively. For high contact pressures, 0.05 wt.% HNTs lowered COF and wear by 55% and 56%, respectively. The load-carrying capacity increased by 141% with 0.10 wt.% HNTs compared to the sunflower oil. A high tapping torque efficiency was obtained with HNTs that can prolong tool life in the machining process. Therefore, this study suggests that HNTs/sunflower oil could be used as green lubricants for industrial applications.

Mussel-Inspired Interfacial Modification for Ultra-Stable MoS2 Lubricating Films with Improved Tribological Behavior on Nano-Textured ZnO Surfaces Using the AACVD Method.

Friction-associated energy loss and mechanical wear leading to failure is a major problem in industries. To mitigate this, the design and testing of novel lubricants is important. Here, we show the facile one-pot synthesis of ZnO/ZnO nanorods (NRs) and MoS2 films on pre-treated glass substrates via aerosol-assisted chemical vapor deposition. The bearing capacity and wear life of ZnO film/ZnO NRs/MoS2 films were improved due to the lubricant retention capabilities of the NRs. Modification of the ZnO/MoS2 nano-arrays using polydopamine (PDA) allowed the realization of robust and ultra-stable solid lubricants through the triple action of chemical chelation, layered materials, and nanotexture, especially under heavy load conditions. Compared with pristine MoS2, the adhesion and bearing strength of the composite film increased by 11 and 30 times, respectively, while the coefficient of friction and wear rate decreased by 94 and 85%, respectively. This is because the chelation between the transition metal and the groups in the interlayer PDA was fully utilized to improve the interface compatibility, which significantly improves the robustness of ZnO NRs and the adhesion of MoS2. This allowed a stable and firm mechanical lock between the substrate, lubricant films, and the steel ball. It demonstrated a convenient method to achieve the antifriction and anti-wear of solid lubricating materials by PDA interface modification for practical industrial applications.

The Development of Triglyceride-Based Additives and Their Lubricity Properties for Low Sulfur Fossil Diesel: A Review

Abstract Reducing the sulfur content in fossil diesel is recognized to create cleaner air. One of the efforts that can be made to reduce sulfur is involving the hydrodesulfurization process in the oil refinery. However, that process is responsible for decreasing the natural lubricating compounds in fossil diesel. Then, it results in the low lubricity of fossil diesel which increases friction and wears scars on the metal surface of machinery components. Therefore, it is necessary to use additives to improve the lubricity properties of fossil diesel. Additives can be derived from vegetable oil-based triglyceride compounds (bio-additives). The primary bio-additives class studied in lubricity improver is the ester group. Many challenges need to be addressed by researchers to obtain such a bio-additive to perform better. This paper focuses on bio-additives and their lubricity properties between 1995 and 2020. We review the bio-additives class and its effect on fossil diesel lubricity, the relationship between structure and lubricity, the bio-additives synthesis route, and the lubricity testing method. Finally, the challenges and future research direction in developing lubricity bio-additives for low sulfur diesel fossil are also highlighted (including the strengths, weaknesses, opportunities, and threats analysis and its strategies briefly), which will be an essential consideration in synthesizing new lubricity improver bio-additives for low sulfur diesel.

Self-Assembled Nanospheres with Enhanced Interfacial Lubrication for the Treatment of Osteoarthritis

Osteoarthritis is associated with an increase in mechanical friction of the joint, which causes irreversible damage to articular cartilage. Consequently, it is crucial to restore joint lubrication for effectively treating osteoarthritis. In the present study, hyaluronic acid (HA)-based zwitterionic nanospheres with phosphocholine groups on the surface were synthesized, which achieved excellent lubrication behavior due to the hydration lubrication mechanism. Specifically, HA was initially thiolated and modified with hexadecylamine based on an amidation reaction, then it was grafted with 2-methacryloyloxyethyl phosphocholine (MPC) by the thiol-ene click reaction, and finally self-assembled into nanospheres (HA-MPC) by hydrophobic interaction and cross-linking of the thiol group. The lubrication test demonstrated that the HA-MPC nanospheres improved lubrication under shear force, with a 40% reduction in the friction coefficient compared with HA. The in vitro experiment indicated that the HA-MPC nanospheres had excellent biocompatibility, and they upregulated the cartilage anabolic gene and downregulated cartilage catabolic proteases as well as the pain-related gene. The in vivo test showed that the injection of HA-MPC nanospheres to the joint cavity could inhibit the development of osteoarthritis, which was examined based on histological staining and also morphological evaluation. In conclusion, the new self-assembled zwitterionic HA-MPC nanospheres may be intra-articularly injected for the effective treatment of osteoarthritis by restoring joint lubrication.

Tribo-functional PVD thin films deposited onto additively manufactured Ti6Al7Nb for biomedical applications

For biomedical applications, the additive manufacturing of titanium-based alloys in combination with a subsequent physical vapor deposition (PVD) of tribo-functional thin films enables producing complex-shaped implants and devices with improved tribological behavior. Titanium nitride (TiN), titanium carbonitride (TiCN), amorphous carbon (a-C), and Ag-containing amorphous carbon (a–C:Ag) thin films were coated on laser powder bed fused (L-PBF) Ti6Al7Nb substrates by magnetron sputtering. TiN exhibits a high adhesion on Ti6Al7Nb, whereas TiCN, a–C, and a–C:Ag have a lower adhesion strength. In lubricated tribometer tests against Al2O3, the PVD thin films are highly effective in improving the tribological properties of additively manufactured Ti6Al7Nb. TiCN, a–C, and a–C:Ag show lower friction than uncoated Ti6Al7Nb and TiN, with a–C and a–C:Ag having the lowest coefficients of friction. Compared to uncoated Ti6Al7Nb, the PVD films also considerably reduce both the wear and counterpart wear.