Underlying Wear Mechanisms Research Articles

Wear Prediction of Functionally Graded Composites Using Machine Learning.

This study focuses on the production of functionally graded composites by utilizing magnesium matrix waste chips and cost-effective eggshell reinforcements through centrifugal casting. The wear behavior of the produced samples was thoroughly examined, considering a range of loads (5 N to 35 N), sliding speeds (0.5 m/s to 3.5 m/s), and sliding distances (500 m to 3500 m). The worn surfaces were carefully analyzed to gain insights into the underlying wear mechanisms. The results indicated successful eggshell particle integration in graded levels within the composite, enhancing hardness and wear resistance. In the outer zone, there was a 25.26% increase in hardness over the inner zone due to the particle gradient, with wear resistance improving by 19.8% compared to the inner zone. To predict the wear behavior, four distinct machine learning algorithms were employed, and their performance was compared using a limited dataset obtained from various test operations. The tree-based machine learning model surpassed the deep neural-based models in predicting the wear rate among the developed models. These models provide a fast and effective way to evaluate functionally graded magnesium composites reinforced with eggshell particles for specific applications, potentially decreasing the need for extensive additional tests. Notably, the LightGBM model exhibited the highest accuracy in predicting the testing set across the three zones. Finally, the study findings highlighted the viability of employing magnesium waste chips and eggshell particles in crafting functionally graded composites. This approach not only minimizes environmental impact through material repurposing but also offers a cost-effective means of utilizing these resources in creating functionally graded composites for automotive components that demand varying hardness and wear resistance properties across their surfaces, from outer to inner regions.

Effects of longitudinal ultrasonic vibration on tool wear behaviors in side milling of GH4169D superalloy

The GH4169D superalloy, a novel Nickel-based material widely acknowledged for its exceptional high temperature strength and fatigue resistance properties, plays a pivotal role in the production of critical components of contemporary aero-engines. The material, however, falls under the category of a typical difficult-to-cut metallic material due to the substantial cutting forces and significant tool wear experienced during the machining process. In this paper, a method of longitudinal ultrasonic vibration assisted side milling (LUVM) was developed to extend tool life. The investigation primarily involved an analysis of tool wear behaviors, including milling force, tool life, morphologies of tool wear, and various underlying wear mechanisms in LUVM and conventional milling (CM) of GH4169D superalloy. The results demonstrate that LUVM exhibits the advantage of reducing milling forces and prolonging tool life in comparison to CM. Throughout the entire milling process, the maximum milling force of LUVM is consistently smaller than that of CM, displaying a comparatively slower growth trend. At the conclusion of machining, while CM reaches a maximum milling force of 360.3 N, the maximum milling force of LUVM (195.9 N) is significantly lower. The tool life of LUVM is 28.2 min, representing a 33 % increase compared to CM (21.2 min). Material adhesion, coating delamination, and built-up edge (BUE) phenomena can be observed on both the tool rake face and flank face in CM. LUVM contributes to mitigation of bonding phenomena, prevention of BUE formation, and avoidance of abrasive wear in comparison to CM. However, it is prone to lead to tool tip chipping. For CM, the tool wear mechanisms include adhesive wear, oxidation wear, abrasive wear and diffusion wear; whereas for LUVM, the tool wear mechanisms comprise of adhesive wear, oxidation wear and diffusion wear.

Effect of in-situ nanoparticles induced by Ti addition on the microstructure and tribological properties of FeCrB alloys

Effect of in-situ nanoparticles (NPs) induced by Ti addition on the microstructure and tribological properties of FeCrB alloys has been investigated systematically by various advanced techniques. The results reveal that the TiC and TiB2 nanoparticles can be in-situ formed when adding Ti into the FeCrB alloys and they show great contribution to the microstructure refinement and tribological performance improvement of FerCrB alloys. With the gradual addition of Ti, the improvement effect of in-situ NPs increases initially and subsequently decreases. The JDF2 alloy (refers to FeCrB alloy containing 1.17 wt % Ti) shows the finest microstructure and the exceptional wear resistance. The average size and fracture toughness of primary borides are only 11.8 % of that in FeCrB matrix alloy (without Ti addition) and increase to 22.7 MPa∙m1/2, respectively. Moreover, the wear rate measures at 3.38 × 10−5 mm3/N·m, which is only 48.8% compared to that of FeCrB matrix alloy. The underlying wear mechanisms of FeCrB alloys have been discussed by unraveling the role of in-situ nanoparticles in the microstructure evolution, which belongs to adhesive wear and abrasive wear.

Effect of HVOF–sprayed nanostructured WC–10Co–4Cr coating on sliding wear and tensile–tensile fatigue properties of TC6 titanium alloy

Aerospace equipment has been placing greater demands on the wear resistance and fatigue strength of titanium alloys. In light of this, the research objective was to prepare a WC-10Co-4Cr coating with better performance through the utilization of the ultra-short distance high-speed oxygen fuel (HVOF) spraying process. Then the microstructure, microhardness, sliding wear, and fatigue performance of TC6 alloy reinforced with WC–10Co–4Cr coating were further explored and analyzed. The ultra–short distance HVOF spraying process was employed to suppress decarburization, resulting in a minimal amount of W2C phase in the coating. The WC–10Co–4Cr coating exhibited excellent surface characteristics, displaying minimal microcracks and a low porosity rate of 0.15 ± 0.08%. The interface between the coating and the substrate was sharp and clean, while the TC6 alloy retained its dual structural characteristics without any microstructural changes. The microhardness of the WC–10Co–4Cr coating exhibited a remarkable value of 1172.64 HV0.025, which is 3.2 times greater than that of the substrate. The coating exhibited a noteworthy decrease in wear rate, wear depth, and wear area, indicating superior wear resistance. Additionally, a thorough examination was conducted to uncover the underlying wear mechanism of the coating under different loads. The WC-10Co-4Cr coating, which was developed through this study, shows a minimal influence on the fatigue strength of the original base material. Furthermore, a comprehensive analysis was conducted to elucidate the factors contributing to the significant fatigue life of the WC–10Co–4Cr coating.

Towards understanding the wear mechanisms of turning with FreeTurn

For many years, tool wear has been a critical issue with respect to productivity, cost, and part quality, particularly for superalloy machining. The underlying wear mechanisms of a new turning tool, FreeTurn, is revealed in this paper towards its potential for a more sustainable way of metal cutting. It is a specially designed turning tool that is held by the tool spindle and has a working plane perpendicular to the centreline of the tool spindle, making it rotatable on its working plane. Owing to the unique feature of FreeTurn compared with conventional turning tools, the focus of this study is on the effect of dynamic tool–workpiece engagement on tool wear. The tool path strategy is first set to achieve a cylindrical surface in longitudinal turning with a rotatable FreeTurn tool. Series of experiments with different tool path strategies are performed on a turn–milling machine, using Inconel 718 as an example of workpiece material. The results indicate that the coupling effect between notch and flank wear is the decisive factor of tool wear in free turning with respect to its rotation and feeding directions. While the tool wear can be significantly reduced if the tool feeds with increasing cutting edge angle, the tool will soon be worn out if it feeds with decreasing cutting edge angle. Tool rotation in free turning plays a key role in reducing adhesion on the cutting tool and increasing the tool rotation speed can further improve this effect. Additionally, a more sustainable solution that uses both sides of the cutting edge becomes possible with FreeTurn as well, improving the tool utilisation. The results demonstrate considerable guiding significance for exploring the potential of this new turning tool towards a longer tool life.

Effect of Temperature and Load on Tribological Behavior in Laser-Cladded FeCrSiNiCoC Coatings.

The FeCrSiNiCoC coatings with fine macroscopic morphology and uniform microstructure were made on 1Cr11Ni heat resistant steel substrate by a laser-based cladding technique. The coating consists of dendritic γ-Fe and eutectic Fe-Cr intermetallic with an average microhardness of 467 HV0.5 ± 22.6 HV0.5. At the load of 200 N, the average friction coefficient of the coating dropped as temperature increased, while the wear rate decreased and then increased. The wear mechanism of the coating changed from abrasive wear, adhesive wear and oxidative wear to oxidative wear and three-body wear. Apart from an elevation in wear rate with increasing load, the mean friction coefficient of the coating hardly changed at 500 °C. Due to the coating's transition from adhesive wear and oxidative wear to three-body wear and abrasive wear, the underlying wear mechanism also shifted.

Effect and optimization of tribological parameters upon wear responses of TiAlN coating

This study investigated the relative effects of tribological process parameters and their optimization for wear responses of a TiAlN thin film deposited on heat-treated and plasma-nitrided novel MDC-K hot work tool steel. The effects of tribological process parameters were studied by conducting a tribological test on the TiAlN thin film against a tungsten carbide (WC) counterbody. The researchers considered three tribological process parameters – namely, sliding velocity, applied load and sliding distance – to conduct the tribological test and study the effect of the parameters on five different wear responses – namely, friction coefficient, surface roughness (R a), wear depth, wear mass loss and hardness. The relative effects of the process parameters were studied using contour plots, and the individual effects and contributions were analyzed using an analysis of variance test. Further, the overall evaluation criteria method was employed to select the optimal settings of tribological process parameters based on the experimental results. Finally, the surface morphology of the worn TiAlN surface against the best and worst parametric settings was evaluated using scanning electron microscopy integrated with energy-dispersive X-ray spectroscopy to investigate the underlying wear mechanisms.

Additive manufacturing SiC-reinforced maraging steel: Parameter optimisation, microstructure and properties

The unique deposition manner of additive manufacturing (AM) allows the near-net-shaping of components with multiple materials configurations and complex geometries, which sheds light on the process of high-performance metal matrix composites (MMCs). This work explores laser powder bed fusion (LPBF) AM of SiC-reinforced maraging steel MMCs to consolidate the merits of both ceramics and metal matrix for improving overall properties. The laser processing parameters were systematically optimised based on the density, roughness and hardness of the deposited samples. The effects of SiC content on the microstructures, mechanical properties, tribological performance, and wear resistance are elucidated. SiC particles are refined with uniform distribution in the metal matrix after laser processing. The highest tensile strength reaches 1611 ​MPa together with an elongation of about 10.1% with 3 ​vol% SiC addition. The tribological performance of MMCs is investigated by studying the coefficient of friction (COF), wear rate, and worn morphology. The COF has been slightly reduced with the SiC addition, and the wear rate of MS reduced from 3.25 ​× ​10 −5 to 1.72 ​× ​10 −5 mm 3 /Nm with the 12 ​vol% SiC addition. The underlying wear mechanisms are also investigated. Besides, the corrosion behaviour of MMCs is also investigated; the addition of SiC (≥6 ​vol%) has improved the corrosion properties of the matrix.

High Wear Resistance of Ultralow-Wear Polyethylene with Different Molecular Weights Under Different Contact Pressure

Ultralow-wear polyethylene (ULWPE), a type of polyethylene homocomposite with excellent wear resistance, has recently been reported; however, its underlying wear mechanism has yet to be clarified. In the current study, three different molecular weights of ULWPE from 205 to 748 kg/mol were experimentally evaluated on a multidirectional motion pin-on-disk wear tester under a contact pressure from 2 to 4 MPa, compared with conventional UHMWPE. The high wear resistance mechanism of ULWPE was investigated with respect to mechanical, microstructural, and surface properties. Compared with UHMWPE, ULWPE had excellent wear resistance. Structure and mechanical characterization showed that the crystallinity and lamellar thickness of ULWPE were significantly higher than those of UHMWPE, which endowed ULWPE with high hardness and strength. Despite its considerably smaller molecular weight than that of UHMWPE, ULWPE still had high interphase content, leading to its superior toughness. The crystallinity, lamellar thickness, Young’s modulus, yield stress, and elongation at break of ULWPE exhibited a downward trend with the increase of molecular weight. Conversely, the interphase content of ULWPE increased with the molecular weight increase. Among all the ULWPE samples, ULWPE with a molecular weight 748 kg/mol had the least wear, as a result of combined both excellent strength and adequate toughness. With an increase in contact pressure, the wear losses of different polymers tended to increase. The wear losses of the least wear ULWPE were 4.71 ± 0.04 mm3/Million Cycles (Mc), 5.11 ± 0.37 mm3/Mc, and 5.77 ± 0.62 mm3/Mc under 2, 3, and 4 MPa. Comprehensive strength and toughness reduced abrasive wear and adhesion wear, endowing ULWPE with excellent wear resistance.

Dependence of fretting wear on the microstructure characteristics and impact on the subsurface stability of a metastable β titanium alloy

The fretting behaviors of a metastable β titanium alloy with three kinds of microstructures were studied. Special attentions have been paid to the underlying wear mechanism mediated by fretting conditions and microstructural features. The wear volume is more sensitive to the stroke amplitude rather than the normal loading. The mixed abrasive-adhesive wear is dominated in all fretting samples. The microstructural features make a significant contribution to the difference in the wear resistance and subsurface microstructure stability. Due to a large difference in the grain size and the ability of plastic deformation in adjacent grains, the SM (single-phase microstructure) sample is more favorable to initiation and propagation of cracks. The DM (duplex microstructure) and BM (basketweave microstructure) samples, however, have a high resistance ability to fretting wear and subsurface microstructure stability. This can be attributed to the large amounts of the β/α interfaces and the dispersed orientation distribution of α precipitates. The present work offers valuable experimental data for the prediction of fatigue life after fretting.

An experimental investigation on milling features of fully-sintered zirconia ceramics using PCD tools

ABSTRACT Zirconia ceramics have emerged as a promising restorative material owing to their superior physical properties and excellent biocompatibility. Among the zirconia family, fully-sintered samples show the highest mechanical properties but the poorest machinability compared with those fabricated at pre-sintering temperatures. The current work aims to study the milling characteristics of fully-sintered zirconia ceramics when using polycrystalline diamond (PCD) tools under varying cutting conditions. The machining responses of fully-sintered zirconia ceramics, such as cutting forces, specific cutting energy, milling temperatures, and surface topographies, were carefully addressed. The obtained results were correlated with the applied milling parameters. The tool wear morphologies were eventually characterized to figure out the underlying wear mechanisms governing the performance of PCD tools. The results indicate that increasing the feed per tooth definitely raises the milling forces and temperatures, while increasing the cutting speed elevates the milling temperatures but decreases the cutting forces. The feed per tooth significantly affects the surface topographies of cut zirconia surfaces due to its effects on the ductile-brittle transition of the workpiece. The PCD tool shows the feasibility of carrying out hard milling of fully-sintered zirconia ceramics, which undergoes a minor degree of wear along its cutting edge.

Rigid three-dimensional networks of hafnium diboride for improving mechanical and wear properties of boron carbide

Being one of the hardest materials known in nature, boron carbide (B4C) suffers from severe degradation of mechanical properties when subjected to external pressure or shear stress, which hinders its application in metal forming and machining industries. Herein, based on the phase diagrams, a new strengthening strategy consisting of constructing three-dimensional hafnium diboride (HfB2) networks was proposed to reinforce B4C using the molten-salts method followed by spark plasma sintering (SPS). It was found that the surface amorphization and subsequent decarburization were the main wear regime of pure B4C. Incorporating hard HfB2 networks in B4C resulted in enhanced mechanical properties and reduced wear rates, as the deformation and decarburization of B4C grains were effectively restrained. The excessively high Hf contents, however, made the composites prone to severe adhesive wear following the material transfer from mating materials during the inception of sliding contact, which led to deteriorated wear performances. Thus, the optimal molar ratio of Hf to B4C was determined with the underlying wear mechanisms delineated. The presented results not only show the feasibility of B4C-HfB2 composites as tool materials but also provide a novel designing strategy employing hard networks rather than conventional compliant binders to fabricate wear-resistant composite materials.

Characterizing tribological behavior of fresh concrete against formwork surfaces

Concrete walls are constructed by pouring fresh concrete in-between formworks. The lifespan of these formworks is limited due to the abrasive nature of concrete. The present work investigates the wear of metallic and polymeric-coated plates in contact with fresh concrete. Two formulations were studied: standard concrete and self-consolidating concrete. A plane-plane tribometer was used to control the fresh concrete friction against the plates. This device reproduces the implementation of concrete in the formwork. Results revealed a Coulomb friction law within the range of tested normal pressures. The friction testing induced changes in the surface topography of the plates. The evolution in topography were associated to two underlying wear mechanisms: a granulate-formwork solid-solid friction and a capillary-dominated friction.

Effect of TiC nanoparticles on microstructural and tribological properties of Cu-TiC nano-composites

The present study has investigated the role of the addition of TiC nanoparticles on the microstructural, mechanical, physical, and metallurgical properties of the Cu-TiC nanocomposites. A Powder metallurgy route was used to fabricate the samples. Cold compaction of powders was done at 480 MPa which was followed by sintering at 950°C. Wt.% of TiC was varied from 0% to 20%. A Pin-On-Disc tribometer was used to carry out the wear and friction tests. SEM and EDS techniques were used to explicate the morphology and microstructures of worn surfaces and to comprehend the underlying wear mechanism. The fabricated samples were investigated against the applied normal load (10, 20, and 30 N), sliding speed (0.5, 1.0, and 1.5 m/s), and sliding distance (900, 1800, and 2700 m). The results revealed that the Hardness and tensile strength were improved by 88.76% and 37.26% respectively due to the addition of TiC and were maximum for Cu-20%TiC while the relative density shows the reverse trend. Further, it was found that wear resistance and coefficient of friction were improved by 87.18% and 51.85% respectively as a function of nano-TiC content. The presence of oxide layers and mechanically mixed layers are detected from worn surface analysis which modulates the tribological behavior of the contact.

Friction and wear behaviour of BN(h) and Ag incorporated nickel phosphorous coatings under dry reciprocating sliding conditions

ABSTRACT Ni–P–BN(h)–Ag hybrid composite coatings were fabricated on the steel substrate by a scalable electroless deposition process. The optimum bath composition and operating conditions were selected to obtain defect-free composite coatings with ∼50 μm thickness. Further, to enhance the hardness, as-deposited coatings were subjected to heat treatment in a tubular furnace at 400°C for 1 h. The dry sliding wear behaviour of the as-deposited (AD) and heat-treated (HT400) composite coatings was evaluated using a linear reciprocating tribometer. The average friction coefficient of the hybrid coating was ∼0.14 (in as-deposited condition) and ∼0.16 (in heat-treated condition); these values represent 51.72% and 52.94% reduction over non-hybrid Ni-P coatings. The specific wear rate of the as-deposited and heattreated composite coatings was found to be 50% and 43% lower, respectively, than that of conventional Ni–P coatings. Detailed wear track analysis was carried out to identify underlying wear mechanisms and tribochemical interactions.

Investigation of the Wear Performance of TiB2 Coated Cutting Tools during the Machining of Ti6Al4V Alloy.

The machining of Ti6Al4V alloy, especially at low cutting speeds, is associated with strong Built-Up Edge (BUE) formation. The PVD coatings applied on cutting tools to machine such materials must have the necessary combination of properties to address such an underlying wear mechanism. The present work investigates and shows that TiB2 PVD coating can be designed to have certain mechanical properties and tribological characteristics that improve machining in cases where BUE formation is observed. Three TiB2 coatings were studied: one low hardness coating and two high hardness coatings with varied coating thicknesses. Wear performances for the various TiB2 coated carbide tools were evaluated while rough turning Ti6Al4V. Tool wear characteristics were evaluated using tool life studies and the 3D wear volume measurements of the worn surface. Chip morphology analyses were done to assess the in-situ tribological performance of the coatings. The micro-mechanical properties of the coatings were also studied in detail to co-relate with the coatings’ performances. The results obtained show that during the rough turning of Ti6Al4V alloy with intensive BUE formation, the harder TiB2 coatings performed worse, with coating delamination on the rake surface under operation, whereas the softer version of the coating exhibited significantly better tool life without significant coating failure.

Influence of natural beach mineral corundum on the wear characteristics of LM30 aluminium alloy composites

The work investigates the effect of corundum (Al2O3) reinforcement on the dry sliding wear of LM30 aluminium alloy composites. Composites with the different size range of corundum particles (fine; 1–20 μm, medium; 32–50 μm and coarse; 75–106 μm) and weight percentages varying from 5 to 20 wt% were prepared. The stir casting process was used for the fabrication of composites. LM30 billets were melted at 850 and stirred with the graphite stirrer for 3–4 min. Corundum particles were added to the melt and the molten mass was stirred for another 3–4 min. Finally, the mixture was poured into a cast iron mould. Pins of 8 mm diameter were prepared for wear test and tested against the hardened die steel disc. Optical microscopy of the composites revealed a uniform distribution of corundum particles in the matrix. The microstructure of the composite also got refined. The acicular morphology of eutectic silicon in base alloy got refined to globular for the composites leading to improvement in the hardness of the composites. Dry sliding wear analysis of the composites revealed the decrement in the wear rate of the composites over the base alloy. Similar to the wear rate, the coefficient of friction of the composites also decreased with the increase in the corundum weight percentage. Finally, the wear tracks and debris analysis of the composites revealed the underlying wear mechanisms during the dry sliding wear behavior at a low contact pressure of 0.2 MPa and high contact pressure of 1.8 MPa. The wear mechanism of the composite is dependent on contact pressure. At low contact pressure of 0.2 MPa, the abrasive type wear transformed to adhesive type at a high contact pressure of 1.8 MPa.

Size effect and micro endmilling performance while sustainable machining on Inconel 718

ABSTRACT Micro endmilling is emerged as a potential micromachining technique in many industries like biomedical, aerospace, optics, electronics and marine, etc. due to the huge demand for micro components with precision. However, relatively higher tool wear and poor surface characteristics are limiting its applicability in many areas. Therefore, the need has been raised to improve the machining characteristics of micro endmilling sustainably. The size effect and micro endmilling performance such as wear behavior, cutting force, and microhardness under dry and sunflower oil based MQL environments while machining on Inconel 718 was studied in this work. Results showed that the sunflower oil based MQL environments have the potential to reduce the size effect region and tool wear significantly. Workpiece adhesion and abrasion on the cutting edge were identified as the main underlying wear mechanism for both conditions. The edge radius, tool flank wear, and cutting forces were found to be reduced by 25%, 30%, and 36% respectively with the application of MQL compared to dry conditions. In addition, an increase of 7% in microhardness with relatively smooth surfaces as well as reduced micro cracks, prows, and voids on the machined surfaces were observed with the application of MQL.

Mechanical and Sliding Wear Performance of AA356-Al2O3/SiC/Graphite Alloy Composite Materials: Parametric and Ranking Optimization Using Taguchi DOE and Hybrid AHP-GRA Method

The novelty of this research work lies in (i) designing and fabrication of hybrid AA356-Al2O3/SiC/Graphite alloy composite materials via semi-automatic stir casting method as per the standard industrial procedure. The ceramic particulates Al2O3/SiC were reinforced in a complementary manner (0–8 wt.%, in the step of 2%) along with constant 3 wt.% of graphite bringing about five alloy composite specimens namely SA-08, SA-26, SA-44, SA-62, and SA-80 respectively. The prepared specimens were explored for any synergistic effect of ceramic combinations on physical (density and void content), mechanical (tensile strength, flexural strength, impact strength and hardness), and sliding wear performance adopting ASTM standards. Simultaneously, optimization of sliding wear process parameters (like sliding velocity, sliding distance, normal load, etc.) using the Taguchi design of experiment and Grey Relational Analysis techniques were explored. Further, surface micrographs were analyzed to understand the underlying wear mechanisms and elemental presence/dispersion over the worn-out surfaces. Thereafter, the ranking of compositions was analyzed using a hybrid AHP-GRA material selection method for optimal selection and recommendations. It has been found that both Taguchi and GRA techniques were effective in parameter optimization and gives an attuned outcome. Composition, SA-44 hybrid alloy composite, having the equal presence of both the ceramic particulates tend to optimize the overall performance criteria. The results of ranking obtained by hybrid AHP-GRA analysis attunes to the subjective analysis.

Microstructures and intrinsic lubricity of in situ Ti3SiC2–TiSi2–TiC MAX phase composite fabricated by reactive spark plasma sintering (SPS)

MAX phase composite Ti3SiC2–TiSi2–TiC based on the Tin+1SiCn system was synthesized by spark plasma sintering (SPS) under vacuum sintering conditions. The microstructural evolution upon synthesis and Vickers indentation contact damage were characterized using scanning electron microscopy (SEM) and optical microscopy (OM). Tribological behaviour of the SPSed MAX phase composite was investigated under dry sliding ambient conditions for evidence of intrinsic lubricity as well as to understand the influence of second phase TiC particles on the wear behaviour of this composite system. Further, the underlying wear mechanisms was elucidated via detailed analyses of the worn surfaces using Raman spectroscopy, SEM-EDS and transmission electron microscopy (TEM). Exhaustive analyses of the worn surface revealed evidence of solid lubrication. Transition in friction and wear is attributed to change in wear mechanism from tribo-oxidative to deformation-induced wear due to the disruption of the tribofilm architecture.