Μελέτη απλών και σύνθετων επικαλύψεων ψυχρού ψεκασμού: μικροδομή, μηχανισμός συναπόθεσης, τριβολογία και αντοχή σε διάβρωση

Abstract

Η έρευνα που πραγματοποιήθηκε κατά την εκπόνηση της διατριβής αυτής αφορούσε τη μελέτη της μικροδομής και του μηχανισμού απόθεσης ή συναπόθεσης επικαλύψεων, απλών (μεταλλικών) και σύνθετων (μεταλλο-κεραμικών), που δημιουργήθηκαν με την τεχνική του ψυχρού ψεκασμού. Επίσης, μελετήθηκε η αντοχή σε διάβρωση και η συμπεριφορά σε τριβή-φθορά των επικαλύψεων ψυχρού ψεκασμού. Ο ψυχρός ψεκασμός (cold gas dynamic spray ή cold spray) είναι η τελευταία εξέλιξη στις τεχνικές θερμικού ψεκασμού. Τα σωματίδια της σκόνης τροφοδοσίας επιταχύνονται σε υψηλότερες ταχύτητες σε σχέση με τις παραδοσιακές τεχνικές ψεκασμού και θερμαίνονται σε θερμοκρασία χαμηλότερη από τη θερμοκρασία τήξης των υλικών της σκόνης και του υποστρώματος. Αρχικά δημιουργήθηκαν επικαλύψεις ψυχρού ψεκασμού τριών διαφορετικών υλικών, που έχουν πληθώρα βιομηχανικών εφαρμογών: Cu, CoNiCrAlY και Ti. Οι επικαλύψεις αυτές μελετήθηκαν ως προς τη μικροδομή τους, ώστε να προσδιοριστεί το υλικό που ήταν καταλληλότερο για χρήση στον ψυχρό ψεκασμό. Επίσης, έγινε σύγκριση των επικαλύψεων ψυχρού ψεκασμού Cu και CoNiCrAlY με τις αντίστοιχες επικαλύψεις συμβατικών τεχνικών ψεκασμού. Οι καλύτερες επικαλύψεις σε σύγκριση με τις επικαλύψεις ψυχρού ψεκασμού των δύο άλλων υλικών, αλλά και σε σύγκριση με τις επικαλύψεις των συμβατικών τεχνικών ψεκασμού, ήταν αυτές του ψυχρού ψεκασμού Cu. Οι επικαλύψεις αυτές ήταν συμπαγείς, δεν περιείχαν οξείδια και η διεπιφάνειά τους με το υπόστρωμα κράματος αλουμινίου (Al2017) δεν παρουσίασε ατέλειες (ρωγμές ή κενά). Επίσης, οι επικαλύψεις ψυχρού ψεκασμού Cu παρουσίασαν υψηλότερη μικροσκληρότητα από τις συμβατικές επικαλύψεις με το ίδιο πάχος. Ο χαλκός αποδείχθηκε ιδανικό υλικό για τον ψυχρό ψεκασμό εξαιτίας της ισχυρής παραμόρφωσής του. Αντιθέτως, από τον ψυχρό ψεκασμό CoNiCrAlY σε υπόστρωμα Hastelloy X και Ti σε υπόστρωμα Ti6Al4V προέκυψαν επικαλύψεις που παρουσίασαν πορώδες και κενά στο εσωτερικό τους και σε τμήματα της διεπιφάνειας επικάλυψης-υποστρώματος. Όμως, ήταν ιδιαίτερα σημαντικό ότι σε κανένα από τα τρία υλικά δε συνέβη οξείδωση κατά τον ψυχρό ψεκασμό. Μετά τη μελέτη των απλών επικαλύψεων, εξετάστηκαν οι σύνθετες επικαλύψεις μήτρας χαλκού ενισχυμένης με σωματίδια Al2O3 και η αντίστοιχη επικάλυψη καθαρού χαλκού. Για το σκοπό αυτό, χρησιμοποιήθηκαν δύο σκόνες Al2O3 με εύρος μεγέθους σωματιδίων 2-12μm και 15-45μm, η κάθε μία από τις οποίες αναμίχθηκε με τη σκόνη Cu για τη δημιουργία των μιγμάτων ψυχρού ψεκασμού. Από τα μίγματα με 10, 20, 25 και 30% κ.β. Al2O3 προέκυψαν επικαλύψεις με περιεκτικότητες Al2O3 3, 5, 6 και 7% κατά επιφάνεια, αντίστοιχα. Τα σωματίδια Al2O3 κατανεμήθηκαν ομοιόμορφα στη μήτρα Cu, οι επικαλύψεις είχαν αμελητέο πορώδες, δεν υπήρχαν ατέλειες στη διεπιφάνεια επικάλυψης-υποστρώματος και δεν παρατηρήθηκε οξείδωση ή μετασχηματισμοί φάσεων κατά τον ψεκασμό. Από την πρόσκρουση των σωματιδίων Cu που δεν αποτέθηκαν, δημιουργήθηκαν κρατήρες. Παρομοίως, τα σωματίδια Al2O3 που προσέκρουσαν, αλλά δεν αποτέθηκαν, δημιούργησαν παραμορφώσεις γωνιώδους μορφολογίας. Επίσης, τα μεγαλύτερα σε μέγεθος κεραμικά σωματίδια που αποτέθηκαν, υπέστησαν θραύση κατά την πρόσκρουσή τους. Από την έντονη παραμόρφωση και πιθανότατα την αυξημένη θερμοκρασία στην περιοχή πρόσκρουσης των σωματιδίων Cu, παρατηρήθηκε εξώθηση υλικού (σχηματισμός “jet”) σε τμήματα της περιφέρειας της περιοχής επαφής τους. Η σύνδεση των σωματιδίων χαλκού, τα οποία παραμορφώθηκαν ισχυρά, ήταν συνδυασμός μηχανικής σύνδεσης και σχηματισμού μεταλλουργικού δεσμού. Τα κεραμικά σωματίδια συνδέθηκαν με τα σωματίδια χαλκού μηχανικά, δηλαδή εγκλωβίστηκαν μεταξύ των όλκιμων σωματιδίων χαλκού. Η προσθήκη των σωματιδίων Al2O3 είχε ως αποτέλεσμα την ελαφρά αύξηση της μικροσκληρότητας των σύνθετων επικαλύψεων σε σχέση με αυτήν της επικάλυψης καθαρού Cu. Από τη συνολική μελέτη των σύνθετων επικαλύψεων, δημιουργήθηκε το φυσικό μοντέλο για τη συναπόθεση των σωματιδίων Cu και Al2O3 και το σχηματισμό των επικαλύψεων ψυχρού ψεκασμού. Η επικάλυψη Ti, οι επικαλύψεις Cu+Al2O3, καθώς και η επικάλυψη καθαρού Cu μελετήθηκαν ως προς τη συμπεριφορά τους σε διάβρωση. Ως ηλεκτρολύτης χρησιμοποιήθηκε διάλυμα 3,5%κ.β. NaCl. Στην επικάλυψη Ti και στο κράμα Ti6Al4V (υπόστρωμα) εφαρμόστηκε η μέθοδος της ανοδικής (ποτενσιοδυναμικής) πόλωσης. Το στρώμα παθητικοποίησης TiO2 που σχηματίστηκε στο Ti6Al4V, ήταν σταθερό σε μεγαλύτερο εύρος δυναμικού σε σύγκριση με αυτό της επικάλυψης Ti. Γενικά, η συμπεριφορά σε διάβρωση της επικάλυψης Ti ήταν περίπου ίδια με αυτήν του συμπαγούς Ti6Al4V. Στις επικαλύψεις Cu+Al2O3 και Cu εφαρμόστηκε αρχικά γραμμική και στη συνέχεια ανοδική πόλωση. Κατά την ανοδική πόλωση οι επικαλύψεις υπέστησαν παθητικοποίηση. Σχηματίστηκαν οξείδια του χαλκού μαζί με αδιάλυτο ένυδρo χλωριούχo άλας του Cu. Όλες οι επικαλύψεις διατήρησαν την εξαιρετική συμπεριφορά σε διάβρωση του Cu. Η περιεκτικότητα και το μέγεθος των σωματιδίων Al2O3 δεν επηρέασε τη συμπεριφορά των επικαλύψεων σε ανοδική πόλωση. Τέλος, εξετάστηκε η τριβολογική συμπεριφορά των επικαλύψεων Cu+Al2O3 (με Al2O3 3, 5 και 7% κατά επιφάνεια και για μέγεθος Al2O3 2-12μm και 15-45μm) και της επικάλυψης καθαρού Cu. Ως ανταγωνιστικό υλικό χρησιμοποιήθηκε σφαίρα Al2O3. Η μελέτη αυτή πραγματοποιήθηκε για ταχύτητες ολίσθησης 5 και 10cm/s, φορτία 2, 5, 7 και 10Ν και 20.000 κύκλους ολίσθησης. Η επικάλυψη Cu σταθεροποιήθηκε σε μικρότερο αριθμό κύκλων ολίσθησης και παρουσίασε χαμηλότερο συντελεστή τριβής από όλες τις επικαλύψεις Cu+Al2O3. Η προσθήκη των σωματιδίων Al2O3 και ιδιαίτερα αυτών με μέγεθος 2-12μm, είχε ευεργετική επίδραση, αφού ο ειδικός ρυθμός φθοράς (mm3/N•m) των σύνθετων επικαλύψεων ήταν μικρότερος από αυτόν της επικάλυψης Cu και στις δύο ταχύτητες ολίσθησης. Ο μηχανισμός φθοράς των επικαλύψεων ήταν η εκτριβή (μικρο-αυλακώσεις και γραμμές μικρο-άρωσης) σε συνδυασμό με πλαστική παραμόρφωση των σωματιδίων Cu και οξείδωση στην πίστα τριβής.

The present thesis is entitled “Study of simple and composite cold sprayed coatings: microstructure, co-deposition mechanism, tribology and corrosion resistance”. Cold Gas Dynamic Spray or Cold Spray is the most recent development in the field of spray techniques for the production of coatings. In this process, the gas (nitrogen, helium or air) is introduced into a Laval type nozzle and produces a supersonic gas flow. Spray particles are accelerated to a high velocity (typically 300-1200 m/s) and are deposited at a temperature well below their melting temperature. Comparing cold spray with conventional thermal spray systems it is obvious that the gas temperature is significantly lower and particles obtain a much higher velocity. Initially in this research, three different materials with important industrial applications were cold sprayed: copper on Al2017 substrate, CoNiCrAlY on Hastelloy X substrate and titanium on Ti6Al4V substrate. The microstructure of these coatings was studied in order to determine the most suitable material for cold spraying. Also, copper and CoNiCrAlY coatings were compared with coatings obtained using conventional thermal spray techniques (High Velocity Oxy-Fuel and Wire Arc). The most suitable material for cold spray was found to be copper. Cold sprayed copper coatings were qualitatively superior, as they were fully dense, they did not contain oxides and the coating-substrate interface did not present any defects (micro-cracks, porosity or voids). More analytically, copper coatings were cold sprayed using four sets of parameters with different values of gas pressure, powder feed rate and number of passes (Table1). Surface roughness of the coatings (Ra) was measured. The increase of gas pressure from 2.0 to 2.5 MPa (comparing coatings A and B) resulted in the slight decrease of Ra, while Ra increased when the powder feed rate increased from 22 to 130g/min (comparing coatings C and D). Concerning the coating thickness, it was found that it was slightly increased when gas pressure increased from 2.0 στα 2.5 MPa. Also, using the same gas pressure, but higher powder feed rate resulted in thicker coating. Table 1: Spray parameters of cold sprayed copper coatings. Coating Gas pressure (ΜPa) Powder feed rate (g/min) Number of passes Α 2,0 22 1 Β 2,5 22 1 C 3,0 22 4 D 3,0 130 4 As it was observed in the cross sections of the coatings, copper particles were heavily deformed. They presented the typical flattened shape, mainly parallel to the coating-substrate interface. High velocity impacts and intense deformation of particles resulted in negligible porosity of coatings. The two dimensions, b (length) and h (height), of the deformed particles in the cross sections were measured. The b/h ratio was 2.4 for all the copper coatings. Also, the particle’s deformability ε, expressed as where d is the particle diameter before impact, was calculated. The ε value was equal to 42% for all the coatings regardless of the spraying parameters. This means that the material, which was copper, was responsible for the intense deformation of the particles. Also, oxidation did non take place during cold spraying, as indicated by the absence of oxides in the cross sections of the coatings, and EDS analyses. During cold spraying, the impacts of the particles caused deformation of the substrate surface. The coating-substrate interface was examined by SEM observation and there were no defects (such as voids, porosity or micro-cracks). In the case of the coating with the highest thickness (coating D, 2080μm), some twins were observed in the cross section. Twins were formed from the cold spraying process and in particular, due to the combined effect of local high strain rate and local temperature rise. These phenomena made recrystallization possible. This theory was also supported from the microhardness measurements. The microhardness values of coatings sprayed using higher number of passes, which consequently obtained higher thickness, were lower. These coatings probably reached a higher temperature due to the increased time of impingement of the heated gas. The later effect controlled the degree of recrystallization in the coatings. Cold sprayed copper coatings were compared to copper coatings sprayed using conventional thermal spray techniques (HVOF and Wire arc). As mentioned, copper particles were not oxidized during cold spray. On the contrary, oxides were observed in the cross sections of thermally sprayed coatings. Also, thermally sprayed coatings presented porosity (HVOF coating: 1.3% and wire arc coating: 17%). Microhardness values of cold sprayed coatings were higher (150HV0.3) compared to thermal spray coatings of the same thickness (HVOF: 123 HV0.3 and wire arc: 110 HV0.3). Moreover, cold sprayed and HVOF coatings were heat treated in 350oC temperature for 1 hour. After this treatment, microhardness of cold sprayed coatings decreased significantly due to the recrystallization. Their microhardness values were lower than that of HVOF coating, because the presence of oxides in HVOF coating hindered its recrystallization. CoNiCrAlY coatings were obtained using cold spray and HVOF spray. Cold spray was performed in substrates with two different types of preparation: grit-blasting and just cleaning. Also, HVOF coatings were obtained using flames with different oxygen/fuel ratio. Cold sprayed coatings presented higher surface roughness (13-15.5μm) than HVOF coatings (6-8μm). The coating thickness was almost equal using the two techniques (cold spray: 150-160μm and HVOF: 110-125μm), but in the case of cold spray coatings it was obtained with lower number of passes (5 passes, while HVOF 10 passes). In the cross sections of cold sprayed coatings, porosity and voids were observed (4.2%) because of the insufficient plastic deformation of the particles. On the contrary, HVOF coatings presented microporosity (negligible and 2.6%). The particles sprayed with both techniques, were deposited almost parallel to substrate surface. The advantage of cold sprayed coatings was the absence of oxides (oxygen: from negligible to 1% wt.). Oxidation took place in HVOF coatings (oxygen: from1.3 to 3.4% wt.) and oxides were observed in the splats boundaries. The HVOF coatings microstructure consisted of partially melted particles, unmelted particles and oxides. HVOF coating-substrate interface was qualitatively better than the cold sprayed coatings-substrate interface. The bonding of cold sprayed coatings with the substrate was not very good, as voids and microcracks were observed. The substrate surface was deformed from the particle impacts. The microhardness values of cold sprayed coatings were 558 HV0.3 and 515 HV0.3 (grit-blasted and not-grit blasted substrates, respectively). Similar measurements were obtained for HVOF coatings (506 and 513 HV0.3 for the two flames used). Titanium particles were cold sprayed and deposited on the Ti6Al4V substrate, despite the lack of intense deformation. A coating of 800μm thickness was formed, but Ti particles were not heavily deformed. Surface roughness Ra was relatively high (20μm) and the coating contained 14% porosity. Porosity included pores of different sizes: small voids between particles and big voids from bridging of the particles. The particles in the coating-substrate interface were deformed in a greater extent than particles near the surface of the coating. This was the result of the subsequent impacts of the particles (shot peening effect). Ti particles impacts caused their slight deformation, but not the deformation of the substrate surface. The microstructure of powder particles was maintained in the coating and oxidation did not take place. Also, the formation of metallurgical bonding at some interfaces between particles as well as between particle and substrate was observed. Considering that copper was the most suitable material for use in cold spray, composite Cu+Al2O3 coatings on Al2017 substrate were studied. These coatings combined the properties of copper matrix and the properties of the reinforcing material (alumina). The aim was to study the microstructural characteristics of the coatings as well as to understand the way that copper and alumina particles were co-deposited. A copper powder of 13μm particle size and two alumina powders, a fine with a size range of 2-12 μm and a coarse of 15-45 μm size range, were used in order to prepare the feedstock mixtures. The copper powder was mechanically blended with each one of the alumina powders in various Al2O3 contents: 0, 10, 20, 25 and 30%wt. The surface roughness of the composite coatings and pure copper coating was in the range of 5-10 μm. From the comparison of coatings which were sprayed using the same number of passes, it was noticed that: a) copper coating presented much higher thickness than composite coatings and b) the increase of alumina content resulted in a slight increase of coating thickness. Porosity of all the composite and pure copper coatings was very low (<1%). This very low percentage of porosity was the result of the tamping effect of the subsequent particle impacts (micro- shot peening). Al2O3 particles were present in the coating-substrate interface and they were uniformly dispersed in the copper matrix of the composite coatings. The Al2O3 content of the coatings depended on the Al2O3 percentage in the feedstock mixture. Spraying with mixtures of 10, 20, 25 and 30%wt. Al2O3 resulted in coatings with 3, 5, 6 and 7% area (and volume) fraction. The %wt. percentage of particles that were deposited, was equal to 10% of the initial percentage in the feedstock mixture. The actual fraction of Al2O3 in coatings was lower than that in the source powder mixture because Al2O3 cannot form a bond with itself. Unless an Al2O3 particle is trapped between adjacent Cu particles it will simply rebound from the coating. The higher percentage of Al2O3 particles of the powder with a size range of 2-12μm that were deposited, presented a mean section diameter of 4 to 6μm. Similarly, in coatings which were sprayed using Cu+Al2O3(15-45μm) mixtures, the comparatively smaller particles were deposited in a higher percentage. More specifically, the mean section diameter of most of the Al2O3 particles was smaller than 20μm. Since alumina particles could not be deformed on impact, some very small gaps (1-2μm) surrounding the hard phase particles were observed. Craters and deformations of angular morphology were observed in the surface of the coatings. They were created from Cu and Al2O3 particles (respectively) that impacted, but rebounded and did not adhere. Also, fragmentation of large Al2O3 particles (for example of 55×15μm2 size) was observed. As the copper particles impinged on the substrate and on the particles previously deposited, a flow of material took place and metal jetting was formed at some parts of interfaces. This formation of jetting was observed in the surface of the coatings as well as in the cross section micrographs. The impact of copper and alumina particles on the substrate deformed its surface and created roughness. The coating-substrate interface was qualitatively very good for all the composite and copper coatings, as no defects were detected (porosity, voids or microcracks). The bonding of coating with the substrate was achieved mainly through the ductile copper particles. However, alumina particles were present in the interface, too. Metallurgical bonding was formed at some interfaces between copper particle and substrate and between copper particles, indicating strong bonding. Al2O3 particles bonded with copper particles mechanically. They were entrapped among the copper particles, which were deformed and surrounded them. Copper particles were heavily deformed and obtained the typical flattened shape almost parallel to the coating-substrate interface. The deformation of copper particles was more intense near their interfaces. Their bonding was based on the heavy deformation and on the formation of metallurgical bonds at some interfaces. Most Cu particle-Cu particle interfaces of the coatings were more attacked by etching than grain boundaries. Only a fraction of particle interfaces formed metallurgical bonding, while the other interfaces were just cold forged. That difference can explain the appearance of particle-particle interfaces after etching. No melting of the sprayed material was observed in the coatings and the microstructure of the powder particles was retained in the coatings. From the results of EDS analysis it was clear that oxidation did not happen during cold spraying, as the oxygen content was negligible. The cold sprayed composite and pure copper coatings presented a relatively high hardness of 163-177 HV0.3 for the various alumina contents, as a result of the high degree of particle deformation and strain hardening. The microhardness values of composite coatings were slightly higher than that of copper coating, due to the reinforcing effect of Al2O3 hard phase. More specifically, the addition of 3% Al2O3 (area percentage) in the coatings resulted in 2-3% increase of microhardness value. The further increase of Al2O3 content at 5, 6 and 7% (area percentage) caused 7 to 9% increase of microhardness values, compared to that of copper coating. The X-ray diffraction patterns of the Cu and Al2O3 powders, of their powder mixtures and of the composite and copper coatings were obtained. Comparing the patterns of the powders to that of the respective coatings, no differences were observed. The only phases which were detected, were Cu and Al2O3. No evidence for the presence of any new phase, such as copper oxides, was found. On the contrary, the XRD analysis of HVOF copper coating revealed not only copper, but also copper oxide Cu2O. These results confirmed the capability of the cold spray process to deposit the feedstock without any phase change or oxidation. The lack of other phases is strong evidence supporting the assumption that the deposition mechanism of cold spray is a solid-state process. The slight broadening of the peaks in the coatings could be attributed to the cold work experienced during the high velocity impacts. The results of the total study of Cu+Al2O3 coatings provided the information for the physical model that was suggested. This physical model describes the phenomena that took place during cold spray and how the composite coating was built. It includes the initiation of spraying, the deposition or the rebounding of the first particles that impinge on the substrate, the formation of the coating-substrate interface, the coating build-up and finally the completion of coating build-up to the desired thickness. The study of corrosion behavior of specimens coated with Ti, Cu+Al2O3 and Cu, was also one of the aims of this research. The electrolyte was 3.5%wt. NaCl solution and the electrochemicals methods of linear and potentiodynamic polarization were applied. Polarization results were studied and the tested surfaces were observed using scanning electron microscopy. From the study of the reactions, Pourbaix diagram and potentiodynamic curves of the cold sprayed Ti coating as well as of the Ti6Al4V alloy (substrate), it was found that TiO2 was formed. In the substrate, the passive layer (oxide) was stable for potential values higher than 0.2V and remained stable up to 1.6V. In the case of the coating, the range of the potential instability was wider compared to that of the substrate alloy. The oxide layer was being formed and destroyed and this was continued for potential values up to 0.4V. The high porosity (14%) of the coating influenced its corrosion behavior and phenomena that took place in the substrate could have been interfered. However, for potential values higher than 0.4V, the stable passive layer was formed and the oxide layer remained stable until 1.4V. At 1.4V potential, the passive layer was dissolved and the transpassive region of the metal begun. Generally, the corrosion behavior of cold sprayed Ti coating was similar to that of bulk Ti6Al4V. The reactions, Pourbaix diagram and polarization curves of Cu+Al2O3 and Cu coatings were studied, too. Initially, linear polarization was applied and after that potentiodynamic polarization was tested. The curves obtained using linear polarization technique presented a very good linear relationship. The values of corrosion potential Ecorr did not present significant differences, as they were in the range of -186 to -197mV for the coatings with different content and size of alumina particles. The potentiodynamic polarization curves of all the coatings coincided regardless of the alumina content and the alumina particle size. This means that the phenomena which happened during the corrosion tests were the same for all the coated specimens. All the potentiodynamic curves of the cold sprayed coatings agreed with the typical curve of Cu, indicating that the cold sprayed copper coatings retained the very good corrosion behavior of bulk copper. From -0.210V (Εi=0) to more positive values of potential, the coatings initially presented anodic drastic behavior. At 0.1V the passivation started and continued up to 1.6V potential (end of potentiodynamic polarization test). The corrosion products were formed in the coatings’ surface in the anodic region of the polarization curves. Copper oxides (Cu2O and CuO), together with insoluble hydrated copper chlorides (CuCl2) were formed during the potentiodynamic polarization. All the coatings presented similar values of corrosion potential (from -212 to -246 mV), corrosion current (from 2.01 to 2.48 x10-5 A/cm2) and corrosion rate (from 9.20 to 11.34 mpy). Considering the corrosion rate values, the corrosion behavior of cold sprayed Cu+Al2O3 and Cu coatings was characterized as “good”. The presence of a green color phase, which was consisted of the corrosion products, was observed at the bottom of the electrolytic cell. The initial morphology of the coatings’ surface was altered, because of the layer which was formed from the corrosion products. This layer covered the copper and alumina particles at the surface of the cold sprayed coatings. The results of EDS analysis revealed the presence of copper, chlorine and oxygen at the surface of the coatings. So, it was confirmed that the corrosion products were copper chloride and copper oxides. Moreover, the tribological behavior of cold sprayed pure copper coating and Cu+Al2O3 coatings which contained 3, 5 and 7% (area percentage) of fine (2-12μm) and coarse (15-45μm) Al2O3 was studied. Tribological tests were performed with a “ball-on-disc” tribometer until 20,000 cycles were completed. The values of sliding velocities were 5 and 10 cm/s and the loads used were 2, 5, 7 and 10N. The antagonistic (counter) material was an Al2O3 ball. The coefficient of friction was continuously recorded and the volume loss was calculated according to ASTM G99-95a Standard using the measured wear track width values. Also, the wear tracks and the antagonistic material were observed by optical microscopy and scanning electron microscopy. Due to the slight differences of the alumina content in the coatings, the coefficient of friction presented slight differences, too. Generally, the values of the friction coefficient μ of Cu and Cu+Al2O3 coatings were in the range of 0.4 to 0.5. The increase of the applied load (2, 5, 7 and 10N) resulted in the slight decrease of the friction coefficient for both sliding velocities. Also, the friction coefficient reached the steady state earlier when the load was increased. Copper coating presented the lowest friction coefficient compared to all the composite coatings (coatings with fine and coatings with coarse Al2O3 particles) for sliding velocities 5 & 10 cm/s. Also the coefficient of friction (μ) of Cu coating reached the steady state earlier than that of the Cu+Al2O3 coatings. Friction coefficient increased with the addition of Al2O3, as ceramic particles increased the friction force. The specific wear rates (mm3/N•m) of all the composite coatings, for all the Al2O3 contents and both Al2O3 particle size ranges, were lower than the specific wear rate of copper coating. These results suggested that the wear resistance of composite Cu+Al2O3 coatings was superior to that of copper coating. Coatings reinforced with fine alumina (2-12μm) particles exhibited lower wear rates than coatings with coarse alumina particles (15-45μm) for both sliding velocities (5 & 10 cm/s). This indicates that the abrasion caused from fine particles (2-12 μm) was slighter than that of the coarse particles (15-45 μm). Also, for both sliding velocities the increase of alumina particles content caused increase of the specific wear rate. When the material that was lost contained large as well as more Al2O3 particles, the result was that the coating was worn in a higher degree. The dominant wear mechanism of the coatings was microabrasion - microploughing. The wear traces exhibited microgrooves and plastic deformation of the copper particles. Also, oxidation of coating material took place during sliding process. Copper and copper oxides were transferred to the antagonistic material, thus changing the initial contact surfaces.