Tribocorrosion characteristics of CrMoSiCN/Ag coatings on Ti6Al4V alloys in seawater

CrMoSiCN/Ag coatings were deposited on Ti6Al4V alloys at the trimethylsilane flow of 15 sccm using closed-field unbalanced magnetron sputtering, and their microstructures were observed and analyzed using SEM, XRD, XPS and TEM, respectively. The coatings’ mechanical properties were measured using nano-indenter. The tribocorrosion characteristics of Ti6Al4V and CrMoSiCN/Ag coatings were investigated in seawater using tribocorrosion tester. The results revealed that the nanocomposite coatings consisted of (Cr, Mo)N solid solution, Ag nanocrystal and amorphous SiCN_x matrix. As the Ag target current increased to 1.0 A, a large amount of Ag nanoparticles were observed on the coating surface. The coating hardness initially increased to 21.0 ± 0.7 GPa at the Ag target current of 0.4 A and then declined. After the Ag element was added into coatings, their tribocorrosion characteristics were improved. The tribocorrosion characteristics of coatings were much better than those of Ti6Al4V. The tribocorrosion characteristics of CrMoSiCN/Ag coating at the Ag target current of 1.0 A were the best in seawater.     

Bias voltage effect on the structure and property of the (Ti:Cu)-DLC films fabricated by cathodic arc plasma

Titanium copper-diamond-like carbon (Ti:Cu)-DLC films were deposited onto silicon via cathodic arc evaporation process using titanium (Ti) and copper (Cu) target arc sources to provide Ti and Cu in the metal containing diamond-like carbon (Me-DLC) film systen. Acetylene reactive gases served as the carbon sources, which were activated at 180 °C at 20 mTorr. Varying substrate bias voltages from − 80 to − 250 V were used in order to provide the (Ti:Cu)-DLC structure. The structure, interface, and mechanical properties of the produced film were analyzed by transmission electron microscope (TEM), IR Fourier transform (FTIR) spectra, and Rockwell C hardness. The process parameters were compared by studying the various mechanical properties of the films, such as microhardness and adhesion. The results showed that the Ti-containing a-C:H/Cu coatings exhibited an amorphous layer of Ti-DLC layer as well as a nanocrystalline layer of Cu multilayer structure. The mechanical properties of the coatings, as determined by Rockwell C indents testing and the ball-on-disc test, varied with the the applied negative DC bias voltage. These (Ti:Cu)-DLC coatings are promising materials for soft substrate protective coatings.     

Synthesis and evaluation of TaC nanocrystalline coating with excellent wear resistance,corrosion resistance,and biocompatibility

To improve the mechanical properties, corrosion resistance, and biocompatibility of implanted titanium alloys, a TaC nanocrystalline coating was deposited on Ti–6Al–4V alloy using a double-cathode glow discharge method. The microstructure of the newly developed coating was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The coating exhibits a dense and uniform structure, composed of equiaxed TaC grains with an average grain size of 15.2 nm. The mechanical properties of the TaC-coated Ti–6Al–4V alloy were evaluated by a scratch tester, a nanoindentation tester, and a ball-on-disc tribometer. The average hardness of the TaC nanocrystalline coating is about 6 times higher than that of uncoated Ti–6Al–4V alloy and the specific wear rate of the coating is two orders of magnitude lower than that for Ti–6Al–4V at applied normal loads of 4.9 N under dry sliding condition. The electrochemical behavior of the TaC nanocrystalline coating after soaking in Ringer's solution for different periods was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Furthermore, in vitro cytocompatibility of the coating was assessed using MC3T3-E1 mouse osteoblastic cells. The results showed that the TaC coating exhibits better corrosion resistance and biocompatibility as compared to uncoated Ti–6Al–4V alloy.     

Electropolymerized polyaniline coatings on aluminum alloy 3004 and their corrosion protection performance

Homogeneous and adherent polyaniline coatings were electrosynthesized on aluminum (Al) alloy 3004 (AA 3004) from an aqueous solution containing aniline and oxalic acid by using the galvanostatic polarization method. A higher applied current density in the polymerization stage proved to be the best condition to adopt for the synthesis of more compact and strongly adherent polyaniline coatings on Al. The corrosion performances of polyaniline coatings were investigated in 3.5% NaCl solution by the potentiodynamic polarization technique and electrochemical impedance spectroscopy (EIS).Potentiodynamic polarization and electrochemical impedance spectroscopy studies reveal that the polyaniline acts as a protective layer on Al against corrosion in 3.5% NaCl solution. The current corrosion decreases significantly from 6.55 μA cm⁻² for uncoated Al to 0.158 μA cm⁻² for polyaniline-coated Al. The corrosion rate of the polyaniline-coated Al is found to be 5.17 × 10⁻⁴ mm year⁻¹, which is ∼40 times lower than that observed for bare Al. The potential corrosion increases from −1.015 V versus SCE for uncoated Al to ∼−0.9 V versus SCE for polyaniline-coated Al electrodes. The positive shift of ∼0.11 V in potential corrosion indicates the protection of the Al surface by the polyaniline coatings.The synthesized coatings were characterized by UV-visible absorption spectrometry, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Optical absorption spectroscopy reveals the formation of the emeraldine form of polyaniline. The results of this study clearly ascertain that the polyaniline has outstanding potential to protect the AA 3004 alloy against corrosion in a chloride environment.     

Preparation,biomimetic apatite induction and osteoblast proliferation test of TiO2-based coatings containing P with a graded structure

TiO₂-based coatings containing P (T–P) were prepared on Ti6Al4V by microarc oxidation (MAO) with applied voltages of 200–400 V in an electrolyte containing (NaPO₃)₆ and NaOH. The surfaces of the T–P coatings became rough and the thickness increased with increasing the applied voltage. Above 200 V, anatase was found on the surface, and rutile was observed at 400 V. With increasing the coating thickness, the O and P concentrations increase; while Ti and Al concentrations decrease. Ti, O and P elements display a uniform distribution character around the micropores on the surface of the T–P coating formed at 300 V. However, the inner of the micropores exhibits a high Ti concentration and low O and P concentrations due to the graded distributions of Ti, O and P elements in the T–P coating. The apatite-forming ability of the T–P coating formed at 300 V was evaluated by immersing in a simulated body fluid (SBF) for 28 and 56 days. The results indicate that biomimetic apatite was formed on the surface of the T–P coating after immersion in SBF for 56 days. And the further cell experiment indicates that the T–P coating can provide surface suitable for the MG63 cell proliferation.     

Electrochemical and structural evaluation of functionally graded bioglass-apatite composites electrophoretically deposited onto Ti6Al4V alloy

Titanium and its alloys are widely used as materials for implants, owing to their corrosion resistance, mechanical properties and excellent biocompatibility. However, clinical experience has shown that they are susceptible to localised corrosion in the human body causing the release of metal ions into the tissues surrounding the implants. Several incidences of clinical failures of such devices have demanded the application of biocompatible and corrosion resistant coatings and surface modification of the alloys. Coating metallic implants with bioactive materials is necessary to establish good interfacial bonds between the metal substrate and the bone. Hence, this work aimed at developing a bioglass-apatite (BG-HAP) graded coating on Ti6Al4V titanium alloy through electrophoretic deposition (EPD) technique. The coatings were characterized for their properties such as structural, electrochemical and mechanical stability. The electrochemical corrosion parameters such as corrosion potential (E_corr) (open circuit potential) and corrosion current density (I_corr) evaluated in simulated body fluid (SBF) have shown significant shifts towards noble direction for the graded bioglass-apatite coated specimens in comparison with uncoated Ti6Al4V alloy. Electrochemical impedance spectroscopic investigations revealed higher polarisation resistance and lower capacitance values for the coated specimens, evidencing the stable nature of the formed coatings. The results obtained in the present work demonstrate the suitability of the electrophoretic technique for the preparation of graded coating on Ti6Al4V substrates.     

A novel biomedical TiN-embedded TiO2 nanotubes composite coating with remarkable mechanical properties,corrosion, tribocorrosion resistance,and antibacterial activity

Tribocorrosion is a severe problem in dental implants, artificial joints, and other implants, and it will affect the long-term safety of the implants. To improve the deficiencies of titanium alloys, we combined physical vapor deposition technology and anodic oxidation to prepare TiN to embed TiO₂ nanotube composite coatings (NTNTs-TiN). Results show that the hardness of the NTNTs-TiN composite coatings reaches 33.2 ± 0.6 GPa, and the grains of the composite coatings were further refined. The NTNTs-TiN coating has the smallest average coefficient of friction (0.22) during tribocorrosion. The tribocorrosion resistance of NTNTs-TiN coating in SBF is increased by ∼44 and ∼2 times compared with Ti6Al4V alloy and TiN coating, respectively. The capillarity effect of the lower contact angle of NTNTs-TiN can form a continuous water-lubrication film at the interface between the counter-ball and coating and produce a lubrication film composed of Ca, Mg, and P, which reduces the coefficient of friction significantly. The NTNTs/TiN composite coating exhibits the best synergistic effect of wear and corrosion. In addition, the NTNTs-TiN coating also exhibits excellent antimicrobial and corrosion properties, which provides a new solution for the long-term safe use of implants in the human body.     

Effect of vitreous enamel coating on the oxidation behavior of Ti6Al4V and TiAl alloys at high temperatures

Vitreous enamel coating is a promising candidate as a high temperature protective coating for titanium (Ti)-based alloys due to its high thermo-chemical stability, compatibility, and matching thermal expansion coefficient to the substrates. Vitreous enamel coating is economically attractive because of its low cost and easy handling. The oxidation behavior of Ti6Al4V (at 700°C) and Ti–48Al (at 800–900°C), with and without the vitreous enamel coating exposed to air, are investigated in this article. The results show that the vitreous enamel coating could markedly protect the substrate (Ti6Al4V and Ti48Al) from oxidation at elevated temperatures. In comparison, the TiAlCr coating might not provide long-term protection for the Ti6Al4V alloys due to the heavy interfacial interdiffusion at high temperatures, although a protective Al₂O₃ scale could form at the initial oxidation stage. The vitreous enamel coating remains intact, uniform, compact, and adhesive to the substrate, however, with undetectable interfacial reaction after oxidation. It is also worth noting that some new phases form in the coating during oxidation at 900°C, although the protectiveness of the coating seems to be unaffected.     

Electrochemical investigation of chromium nanocarbide coated Ti–6Al–4V and Co–Cr–Mo alloy substrates

This study investigated the electrochemical behavior of chromium nano-carbide cermet coating applied on Ti–6Al–4V and Co–Cr–Mo alloys for potential application as wear and corrosion resistant bearing surfaces. The cermet coating consisted of a highly heterogeneous combination of carbides embedded in a metal matrix. The main factors studied were the effect of substrate (Ti–6Al–4V vs. Co–Cr–Mo), solution conditions (physiological vs. 1 M H₂O₂ of pH 2), time of immersion (1 vs. 24 h) and post coating treatments (passivation and gamma sterilization). The coatings were produced with high velocity oxygen fuel (HVOF) thermal spray technique at atmospheric conditions to a thickness of 250 μm then ground and polished to a finished thickness of 100 μm and gamma sterilized. Native Ti–6Al–4V and Co–Cr–Mo alloys were used as controls. The corrosion behavior was evaluated using potentiodynamic polarization, mechanical abrasion and electrochemical impedance spectroscopy under physiologically representative test solution conditions (phosphate buffered saline, pH 7.4, 37 °C) as well as harsh corrosion environments (pH ～ 2, 1 M H₂O₂, T = 65 °C). Severe environmental conditions were used to assess how susceptible coatings are to conditions that derive from possible crevice-like environments, and the presence of inflammatory species like H₂O₂. SEM analysis was performed on the coating surface and cross-section. The results show that the corrosion current values of the coatings (0.4–4 μA/cm²) were in a range similar to Co–Cr–Mo alloy. The heterogeneous microstructure of the coating influenced the corrosion performance. It was observed that the coating impedances for all groups decreased significantly in aggressive environments compared with neutral and also dropped over exposure time. The low frequency impedances of coatings were lower than controls. Among the coated samples, passivated nanocarbide coating on Co–Cr–Mo alloy displayed the least corrosion resistance. However, all the coated materials demonstrated higher corrosion resistance to mechanical abrasion compared to the native alloys.     

Novel structured spark plasma sintered thermal barrier coatings with high strain tolerance and oxidation resistance

Titanium alloys play an important role in lightweight aircraft engines owing to their low densities and high specific strengths. However, an increase in the thrust-to-weight ratio causes the engine operating temperature to be much higher than the service temperature, which deteriorates the oxidation resistance and mechanical properties. In this study, yttria-partially stabilised zirconia (8YSZ)/NiCrAlY thermal barrier coatings (TBCs) with a bimodal structure were prepared on Ti–6Al–4V by using spark plasma sintering (SPS) to improve the service temperature. The distinctive bimodal structure possessed dense particle contacts and a uniform distribution of porous nanoparticles, resulting in higher strain tolerance, sintering resistance, and lower thermal conductivity. Therefore, the bimodal structure prepared by lowering the SPS preparation temperature increased the high-temperature service time of TBCs on titanium alloy. The ceramic top coating (TC) and bond coating (BC) were well connected after isothermal oxidation at 800 °C for 100 h. The TBCs only shed 6% of their surface area at high temperature and large-angle bending. In addition, the bimodal-structured TBCs effectively improved the oxidation resistance of the Ti–6Al–4V substrate. The Ti–6Al–4V substrate with bimodal-structured TBCs only gained 0.51 times the mass gained by the bare Ti–6Al–4V after 100 h of isothermal oxidation.     

Characterization and corrosion behavior of plasma sprayed calcium silicate reinforced hydroxyapatite composite coatings for medical implant applications

Titanium and its alloys are widely used for medical implant applications, but their corrosion in the physiological environment leads to the discharge of metal ions, which can trigger severe health issues. In the present study, calcium silicate reinforced hydroxyapatite (HA-CS) coatings were deposited on the Ti6Al4V substrate by using atmospheric plasma spray (APS) process with an aim to improve the corrosion resistance and bioactivity. The coatings were prepared by varying the weight percentage (wt %) of calcium silicate (CS) reinforcement in hydroxyapatite (HA) as Ha/x CS (x = 0, 10, 20 wt %). The SEM analysis of the pure HA coating revealed the presence of surface microcracks, whereas HA-CS coatings displayed the crack-free surface morphology. The corrosion investigation revealed that with the progressive increment of CS content in HA coating, the corrosion resistance of HA-CS coatings improved. In addition, surface roughness, porosity, microhardness and crystallinity increased with the increase of CS content in HA. The findings of this study indicate that the development of plasma sprayed HA-CS coatings is a promising approach to improve the performance of Ti6Al4V alloy for medical implant applications.     

Effects of powder size and metallic bonding layer on corrosion behaviour of plasma-sprayed Al2O3-13% TiO2 coated mild steel in fresh tropical seawater

This study focuses on the effects of powder size and Ni–Al bonding layer on the electrochemical behaviour of plasma-sprayed Al₂O₃-13% TiO₂ coating in fresh tropical seawater. The presence of the metallic bonding layer reduces the coating porosity and increases the surface roughness for both microparticle and nanoparticle coatings. The nanoparticle exhibits better corrosion rate of 1.9×10⁻⁶ mmpy compared to the microparticle coating, with a corrosion rate of 3.05×10⁻⁶ mmpy. However, the presence of the metallic bonding layer increases the corrosion rate for both micro and nanoparticle coatings. The corrosion mechanism for the coating with and without the metallic bonding layer is discussed in detail.     

Preparation and tribocorrosion performance of CrCN coatings in artificial seawater on different substrates with different bias voltages

The CrCN coatings have been prepared by multi-arc ion plating technology with different bias voltages on 316?L, TC4 and H65 substrates, respectively. The prepared CrCN coatings have been characterized by XRD, SEM, and EDS, respectively. The mechanical properties, electrochemical corrosion behavior, and tribological performance of prepared coatings were tested by microhardness tester, scratch tester, electrochemical workstation, and friction and wear tester, respectively. Results show that the CrCN coatings with bias voltage of ?50?V presented the finer grain size, denser structure, better comprehensive mechanical properties and friction, and better corrosion resistance than the CrCN coatings with a bias voltage of ?30?V. The coating on TC4 substrate show the lower hardness, the better adhesion, the better electrochemical properties and tribological properties than that on 316?L substrate. The coatings based on H65 Cu substrate presented the worst electrochemical and wear properties. The CrCN coating with a bias voltage of ?50?V on TC4 substrate is an optimal candidate in artificial seawater for tribocorrosion.     

Electrochemistry of TiF4 in 1-butyl-2,3-dimethylimidazolium tetrafluoroborate

Electrochemical behaviour of Ti(IV) species in the ionic liquid (IL) 1-butyl-2,3-dimethylimidazolium tetrafluoroborate (BMMImBF₄) was studied by means of chronopotentiometry (CP) and cyclic voltammetry (CV) in melts with different concentrations of TiF₄ (2-35 mol%) within a temperature range of 65-180 °C. The electrochemical reduction of Ti(IV) was suggested to proceed via the sequence of one-electron steps with the formation of poorly soluble low valence intermediates (LVI). LVIs undergo further solid-state electrochemical reduction to Ti metal. Thin Ti coatings on a Pt substrate were thus obtained and characterized by ESEM method. FTIR spectroscopy was used for identification of the electrochemical active species of Ti(IV). The reaction 2BF₄⁻ + TiF₄ ⇔ TiF₆²⁻ + 2BF₃↑ takes place in the concentrated solutions of TiF₄ at elevated temperatures.     

Deposition behavior and characteristics of hydroxyapatite coatings on Al2O3, Ti,and Ti6Al4V formed by a chemical bath method

A simple chemical bath method was used to deposit hydroxyapatite (HA) coatings on Al₂O₃, Ti, and Ti6Al4V substrates at ambient pressure by heating to 65–95 °C in an aqueous solution prepared with Ca(NO₃)₂·4H₂O, KH₂PO₄, KOH, and EDTA. The deposition behavior, morphology, thickness, and phase of the coatings were investigated using scanning electron microscopy and X-ray diffractometry. The bonding strength of the coatings was measured using an epoxy resin method. The HA coatings deposited on the three kinds of substrates were fairly dense and uniform and exhibited good crystallinity without any additional heat treatment. A coating thickness of 1–1.8 μm was obtained for the samples coated once. By repeating the coating process three times, the thickness could be increased to 4.5 μm on the Al₂O₃ substrate. The bonding strength of these coatings was 18 MPa.     

Development of an environmentally friendly protective coating for the depleted uranium-0.75 wt.% titanium alloy: Part I. Surface morphology and electrochemistry

The surface of the depleted uranium (DU)-0.75 wt.% titanium alloy has been studied using scanning electron microscopy, energy dispersive spectroscopy and optical microscopy. The samples were examined after mechanical polishing and again after nitric acid cleaning. The acicular martensitic microstructure is revealed after chemical etching. Several of the impurities have been identified and their prevalence has been found to change depending on the surface treatments. The impurities have also been found to vary from sample to sample and within the same sample.The electrochemistry and corrosion characteristics of the alloy were studied using open circuit potential measurements and potentiodynamic polarization techniques. This study has been directed towards developing environmentally friendly protective coatings for this alloy. In this paper, we discuss our efforts in finding suitable chemical species to act as inhibitors and activators during coating formation. The effect of various oxyanions, MoO₄²⁻, PO₄³⁻, VO₄³⁻, MnO4⁻, SiO₄⁴⁻ and WO₄²⁻, on the electrochemical behavior of the depleted uranium alloy in quiescent nitric acid has been explored including their ability to inhibit corrosion. Results indicate that chemical or electrochemical activation of the DU alloy in 0.1 M HNO₃ + 0.025 M MoO₄²⁻ can lead to the formation of a rudimentary coating. The effect of several fluorine compounds was also examined and their electrochemical response indicates that several of them may have a potential use as a surface activator.     

Effect of microstructure on the adhesion strength of TiBx coating on Ti6Al4V substrate

TiB_x coatings were deposited on Ti6Al4V and Si (100) wafer substrates by D.C. magnetron sputtering with various target-to-substrate distances (T.S. distances) from 50 mm to 200 mm. The influence of T.S. distance on the microstructure, hardness and adhesion strength of TiB_x coatings and Ti6Al4V substrate system was investigated. Results showed that the microstructure of TiB_x coatings transformed from dense to fibre columnar grain with the increase in T.S. distance, whilst the hardness decreased from 20.9 GPa to 9.4 GPa. The Rockwell-C indentation adhesion strength grade was also improved from HF6 to HF1. An adhesion evaluation factor G, which is related to the mechanical properties and the microstructure of TiB_x coating, is proposed based on the test results. The adhesion strength increased with G, which corresponded well with the results of indentation test. The high-speed rubbing test with a sliding speed of 300 m/s was performed to check the Al-adhesion resistance of the TiB_x coating against Al–hBN seal coating.     

Corrosion behaviour of plasma sprayed graphene nanoplatelets reinforced hydroxyapatite composite coatings in simulated body fluid

Hydroxyapatite (HA) coatings, reinforced with varied concentration (0–2 wt%) of Graphene nanoplatelets (GNPs) have been deposited on titanium alloys (Ti–6Al–4V) substrate using atmospheric plasma spraying. Present work studies the effect of GNP concentration on the electrochemical behaviour of the HA coatings in simulated body fluid (SBF). The HA coating exhibited 15% porosity, whereas reinforcement of 1 wt% GNPs in HA (HA-1G) shows 13% porosity, further addition of 2 wt% GNPs in HA reduced the porosity to 10%. Reduction in porosity was achieved as GNPs easily accessed the inter-lamellae to fill the gaps at inter splat region and minimized the occurrence of post-plasma spray defects such as porosity, voids, microcracks etc. These consequences nextward resulted in the significant enhancement in corrosion resistance of the matrix. HA-1G displayed a significant reduction by 67% in the corrosion rate in SBF solution, while this reduction came to 87% for HA-2G coatings. Randomly oriented wrinkles in the GNPs after corrosion process and their hydrophobic nature effectively hindered the SBF infiltration into the coating and resisted their movement towards the underlying substrate. This in turn improved the overall corrosion resistance of the system.     

In situ TiBX/TiXNiY/TiC reinforced Ni60 composites by laser cladding and its effect on the tribological properties

Ni-based composite coatings reinforced by TiB_X/Ti_XNi_Y/TiC with different Ti6Al4V contents were precipitated on a 35CrMoV substrate via laser cladding. The phase composition, elemental distribution, and precipitated phases of the coatings were characterised using X-ray diffraction, energy dispersive X-ray spectroscopy, scanning electron microscopy, and transmission electron microscopy. The mechanical and tribological properties of the cladding layer were also characterised. The results showed that the coating contained TiB₂, TiC, TiB, Ni₃Ti, and NiTi₂ phases with uniform elemental distribution and grain refinement. A schematic of the growth model and precipitation sequence of the reinforced phases was generated. The microstructure, elemental segregation, hardness, and friction behaviour of the cladding layer were significantly influenced by the addition of Ti6Al4V. The optimal microstructure and best mechanical properties were obtained by the addition of 4 wt% Ti6Al4V, with that coating possessing a hardness, average friction coefficient, and wear volume of 770.8 HV₁, 0.180 and 6132 um³, respectively.     

Establishing relationships between bath chemistry, electrodeposition and microstructure of Co-W alloy coatings produced from a gluconate bath

Electrodeposited Fe group: W and Mo alloys have the potential to replace hard Cr coatings for use in engineering applications where wear and corrosion resistance are needed. Electrochemical studies have concentrated in the past on Ni-W alloy deposition, but now interest in Co-W alloys has developed as they possess lower coefficients of friction when in contact with another metal. The most attractive coating composition is in the range 14-20 at.% W, if controlled deposition promotes crystalline alloys of high hardness, rather than softer amorphous alloys containing >20 at.% W. This paper employs ammonia free baths with low concentrations of cobalt and sodium tungstate and varying additions of sodium gluconate to produce alloys at close to 50% efficiency. Voltammetry, UV and visible spectrometry, and potentiostatic deposition have been performed on such baths, whilst XRD, SEM and TEM observations have been made on the deposits. This aims to optimise the process and to understanding the relationships between bath contents, electrochemical kinetics and alloy composition. Efficient deposition of coatings with hardness values up to 1000 kgf mm⁻² occurred from a bath containing a high concentration of gluconate. Such deposits arise from concentrations of Co-W-gluconate complexes which promote the formation of nanoscale alloy grains. Current densities up to 2.75 A dm⁻² in the agitated bath promoted deposition kinetics to form these highly orientated structures. These kinetics produced nano-segregation of W which may be assisted by the migration of Co-W clusters to boundary sites during the growth of the deposit.