喷砂预处理对HVOF喷涂TiAl-Nb/NiCrAl涂层结合强度的影响

采用不同的喷砂压力对基体表面进行喷砂预处理,研究了基体表面状态的变化对HVOF喷涂TiAl-Nb/NiCrAl涂层结合强度的影响。结果表明：随着喷砂压力的增大,基体粗糙度及表面凹坑的深度和宽度增大,NiCrAl层与基体结合界面处孔洞等缺陷增多,同时基体表面残余砂粒的面积分数增加;涂层结合强度随基体粗糙度的增大,先增大后减小,当基体粗糙度为8.33μm时,结合强度达到最大值44.5 MPa。     

Examination of a grit-blasting process for thermal spraying using statistical methods

An experimental study was conducted to develop an understanding of how the grit blasting process, prior to plasma spray coating, affects various properties of the substrate and coatings. A statistical design of experiment approach was used and the results were analyzed using both the linear regression method and average response of factors calculations. The following process variables were studied: grit size (20, 36, 54), blasting pressure (20, 35, 50 psi), blasting duration (4, 6, 8 passes), blasting distance (4, 6 in.), and blasting angle (45°, 90°). Properties such as bond strength, grit contamination, surface roughness, and substrate distortion were evaluated and correlated to the process variables. Based on multiple linear regression results, it was shown that the bond strength can be improved by increasing all of the parameters within the range studied here. No relationship between the surface roughness and bond strength was observed. Grit contamination is mostly influenced by grit size, blasting pressure, and number of blasting passes. The average response method provided indications to the direction of modifying the required properties as a function of process variables. While the average response method agreed mostly with the linear regression predication, some differences are further discussed in the study.     

Effects of blasting parameters on removability of residual grit

This article shows the quantitative evaluation of the residual grit on a blasted substrate, and the removability of the residual grit is examined. Carbon steel plates were blasted by white alumina grit with mean diameters of 338 to 1106 μm. The velocity and the number of grit particles were measured during blasting. The residual grit was removed from a substrate surface by the dissolution of the blasted substrate surface. A mixed acid solution was used as the dissolution solution. The residual grit weight was 7 to 17 g/m². The amount of the residual grit and the penetration depth of the embedded grit increased with increasing grit size. The penetration depth was 5 to 9% of the mean diameter of the grit. The residual grit weight and the penetration depth increased with the increase of the momentum of the grit particle. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Effect of grit blasting and spraying angle on the adhesion strength of a plasma-sprayed coating

A study of the effects of grit-blasting and plasma-spraying angles on the adhesion strength of an alloy (Tribaloy 800) that was plasma sprayed on a titanium-base alloy is reported. Five different spray and grit-blast angles were investigated: 45°, 55°, 65°, 75°, and 90°. The surface texture in different directions was characterized by the classic average roughness and by a fractal analysis number using a two-dimensional fractal analysis method. The grit residue was measured by an x-ray spectrometer. The study showed that the maximum adhesion strength was close to a 90° blasting and spraying angle. However, the grit residue reaches its maximum at a 75° blasting angle. From the image analysis of the interface in different directions, it was found that the nonperpendicular grit blasting produces an anisotropic surface. The fractal analysis method showed a rather good correlation with the blasting angle. However, no good correlation between the fractal number and the adhesion strength was found.     

Stress corrosion cracking of gas pipelines - Effect of surface roughness, orientations and flattening

The primary corrosion mitigation of the external surface of high pressure steel gas pipelines is protective coatings with secondary protection usually by cathodic protection. Adhesion and resistance to cathodic disbondment of the coating is critical for its integrity and grit blasting is an important process in achieving this adhesion. The effect of surface roughness, from grit blasting, on the intergranular stress corrosion cracking resistance of X70 gas pipelines was investigated using slow strain rate testing in carbonate/bicarbonate solution at 75 °C. The effect of orientation of test pieces with respect to the axial direction of pipes was also investigated.Time to failure ratios decreased with increasing surface roughness indicating reduced stress corrosion cracking resistance. The reduced resistance to cracking with increasing roughness would be predominantly associated with stress concentration effects related to the surface roughness resulting from the grit blasting. Crack concentration decreased with increasing roughness, which is likely to be associated with the concentration of surface damage from the grit blasting using varying sized grit. As formed pipe surfaces, with no grit blasting, resulted in some of the lowest time to failure ratios and hence some of the lowest resistances to stress corrosion cracking. These also showed some of the deepest cracks. The influence of roughness and residual stresses on threshold stress is currently being investigated.Time to failure ratios indicated a greater resistance to stress corrosion cracks for circumferentially orientated test pieces compared to those longitudinally orientated. Whilst further testing would be required for confirmation, the current results suggest that flattening the test pieces had only a minor, if any, effect on stress corrosion cracking susceptibility as measured by slow strain rate testing to fracture.     

Fatigue behavior of a 4140 steel coated with a NiMoAl deposit applied by HVOF thermal spray

The fatigue behavior of a quenched and tempered AISI 4140 steel has been investigated in three different conditions: as-polished, as-grit blasted with Al₂O₃ particles and as-coated, after grit blasting, with a deposit of Ni–Al–Mo alloy (Metco 447) of approximately 300 μm in thickness, applied by HVOF thermal spraying. It has been determined that after grit blasting with particles of 20 mesh (83 μm) at a pressure of 345 kPa, a significant decrease in the fatigue properties of the material takes place. It has also been observed that such particles, are retained at the substrate surface during blasting and become stress concentrators that enhance the nucleation of fatigue cracks. The latter give rise to a decrease in the fatigue strength of the blasted material. Further coating of the grit blasted specimens with a deposit of Metco 447 of approximately 300 μm thick, applied by HVOF thermal spraying, leads to a further reduction in the fatigue strength of the material. Under these conditions, the fatigue cracks are also nucleated at the alumina particles retained after blasting. It is believed that such a further decrease is mainly associated with two different causes. Firstly, the extensive fracture and delamination of the coating from the substrate which has been observed from the microscopic analysis. Secondly, the possible existence of tensile residual stresses in the substrate, in the vicinity of the substrate–deposit interface, which would assist in the propagation of the fatigue cracks nucleated at the alumina particles. The fatigue properties of the steel substrate in the three different conditions investigated, has been described in terms of the simple parametric relationship earlier proposed by Basquin.     

Effect of grit-blasting on substrate roughness and coating adhesion

Statistically designed experiments were performed to compare the surface roughness produced by grit blasting A36/1020 steel using different abrasives. Grit blast media, blast pressure, and working distance were varied using a Box-type statistical design of experiment (SDE) approach. The surface textures produced by four metal grits (HG16, HG18, HG25, and HG40) and three conventional grits (copper slag, coal slag, and chilled iron) were compared. Substrate roughness was measured using surface profilometry and correlated with operating parameters. The HG16 grit produced the highest surface roughness of all the grits tested. Aluminum and zinc-aluminum coatings were deposited on the grit-blasted substrates using the twin-wire electric are (TWEA) process. Bond strength of the coatings was measured with a portable adhesion tester in accordance with ASTM standard D 4541. The coatings on substrates roughened with steel grit exhibit superior bond strength to those prepared with conventional grit. For aluminum coatings sprayed onto surfaces prepared with the HG16 grit, the bond strength was most influenced by current, spray distance, and spray gun pressure (in that order). The highest bond strength for the zinc-aluminum coatings was attained on surfaces prepared using the metal grits.     

Peening Effect of Thermal Spray Coating Process

The present study investigated the influence of grit blasting, feedstock powder, and thermal spraying technology on performance near the surface on the substrate’s side. The experimental results show that both the grit-blasting process and thermal spraying process harden the substrate, and microhardness on or near the surface was noticeably increased. Grit blasting created deformed regions next to the surface of the substrate and interface between entrapped grits and substrate. Initial equiaxed grains in the deformed regions were elongated and spirally oriented surrounding impact spots. There were no visible changes in microstructure caused by thermal spraying, and the elongated grain regions remained in the coated substrate. Substrate hardening was attributed to grit blasting and associated heating due to flame rather than powder particle impacting during thermal spraying, thus feedstock powder and individual thermal spray technology had no influence on the hardening.     

Residual stress development in cold sprayed Al,Cu and Ti coatings

Residual stresses play an important role in the formation and performance of thermal spray coatings. A curvature-based approach where the substrate–coating system deflection and temperature are monitored throughout the coating deposition process was used to determine residual stress formation during cold spray deposition of Al, Cu and Ti coatings. The effect of substrate material (carbon steel, stainless steel and aluminium) and substrate pre-treatment (normal grit blasting, grit blasting with the cold spray system and grinding for carbon steel substrate) were studied for all coating materials with optimized deposition parameters. Mainly compressive stresses were expected because of the nature of cold spraying, but also neutral as well as tensile stresses were formed for studied coatings. The magnitudes of the residual stresses were mainly dependent on the substrate/coating material combination, but the surface preparation was also found to have an effect on the final stress stage of the coating.     

Microstructure and fatigue behavior of cold spray coated Al5052

The effect of cold spray coating in inducing residual stresses in the substrate and its effect on delaying crack initiation under cyclic loading have been studied on Al5052 alloy specimens. Different sets of Al5052 specimens have been coated with pure Al and Al7075 feedstock powder, using a low-pressure cold spray coating technique. Some sets of specimens were grit blasted (GB) before coating. The microstructural evolution of the substrate after coating and the fatigue behavior of the coated structure have been studied. In order to obtain the fatigue S–N diagram for each set, as-received and coated specimens with and without preceding GB treatment have been tested in a load-controlled condition. X-ray diffraction has been used to measure the residual stresses both in the deposited materials and the substrates. The results are discussed to highlight the effect of this emerging surface treatment on the characteristics of the treated material. Compressive residual stresses, which led to appreciable increase in the fatigue life, have been observed in all the coated sets. The results indicate that the fatigue strength was significantly improved up to 30% in the case of Al7075 coatings. The results show a strong dependency of the fatigue strength on the deposited material and the spray parameters.     

Loose abrasive blasting as an alternative to slurry polishing of optical fibre end faces

A novel method of polishing the end face of an optical fibre by blasting it with loose dry abrasive grit travelling at high speeds is presented. The method, called loose abrasive blasting (LAB), is specifically designed to improve the surface quality of micro-lenses ground at the tips of optical fibres. Thus, the method described is suitable for polishing non-flat surfaces. Blasting is carried out by immersing the fibre tip in a stream of high kinetic energy abrasive grit. The surface finish attainable using LAB is compared with that of cleaved, ground and slurry polished fibre end faces. Optical microscopy photographs are presented as a qualitative comparison. The surface roughnesses are measured using atomic force microscopy (AFM). For cleaved fibres, the surface roughness improved by a factor of 2 and 6 for slurry polished and dry diamond blasted fibres, respectively.     

Fatigue behavior of a SAE 1045 steel coated with Colmonoy 88 alloy deposited by HVOF thermal spray

An investigation has been conducted in order to study the fatigue behavior of a SAE 1045 steel substrate coated with a Ni-base alloy known commercially as Colmonoy 88, deposited by HVOF spray technique. Fatigue tests were conducted under axial conditions (R = 0.1), employing samples of the substrate material in the as-polished condition, after grit blasting with alumina particles and after grit blasting and coating with a deposit of about 250 μm thick. The fatigue tests were conducted at maximum stresses in the range of 380-533 MPa, depending on the condition of the material. A detailed fractographic analysis of some selected samples tested at different stresses was carried out, aimed mainly at determining the crack nucleation and propagation sequence. The results indicate that the deposition of such a coating leads to a fatigue strength debit of the substrate in the range of 10-20% and a similar debit in fatigue limit of ∼ 11-13%. It has been found that grit blasting is the process responsible for the fatigue strength debit observed in the coated samples. Fatigue cracks have been observed to initiate at the substrate-coating interface and at the free surface of the coating, mainly close to alumina particles embedded on the substrate and sharp notches produced during the process. The fractographic analysis of the fracture surface of the coated specimens points out the characteristic heterogeneous nature of the coating, particularly regarding some of its mechanical properties, such as fracture toughness.     

Deposition and characterization of flame-sprayed aluminum on cured glass and basalt fiber-reinforced epoxy tubes

An oxy-acetylene flame spray torch was used to deposit thin layers of aluminum onto cured glass and basalt fiber-reinforced epoxy tubes. The composite specimens were fabricated by filament winding. Surface coatings embedded in composite laminates were produced. The composite substrates were grit blasted to promote adhesion of the molten aluminum particles. It was found that adhesion increased significantly when the composite substrate was lightly grit blasted, with no adhesion on smooth composite surfaces. The number of passes of the flame spray torch was varied to change the coating thickness and uniformity over the substrate. The electrical resistance of the coatings was measured to assess the suitability of a coating as a conductor. It was found that uniform, electrically conductive coatings were produced with a minimum of two torch passes. Optical images were captured to characterize the coating microstructure and thickness. This investigation did not reveal any visible evidence of damage to the composite substrate. To assess possible degradation effects from the grit blasting and flame spraying processes, the tube specimens were subjected to mechanical testing by applying internal pressurization with hydraulic oil. The tests indicated that the grit blasting and flame spraying processes must be carefully executed to mitigate degradation of the strength of the composite material substrate.     

Characterization of blasted austenitic stainless steel and its corrosion resistance

It is known that the corrosion resistance of stainless steel is deteriorated by blasting, but the reason for this deterioration is not clear. A blasted austenitic stainless steel plate (JIS-SUS304) has been characterized with comparison to the scraped and non-blasted specimens. The surface roughness of the blasted specimen is larger than that of materials finished with #180 paper. A martensite phase is formed in the surface layer of both blasted and scraped specimens. Compressive residual stress is generated in the blasted specimen and the maximum residual stress is formed at 50–100 μm from the surface. The corrosion potentials of the blasted specimen and subsequently solution treated specimen are lower than that of the non-blasted specimen. The passivation current densities of the blasted specimens are higher those of the non-blasted specimen. The blasted specimen and the subsequently solution treated specimen exhibit rust in 5% sodium chloride (NaCl) solution, while the non-blasted specimen and ground specimen do not rust in the solution. It is concluded that the deterioration of corrosion resistance of austenitic stainless steel through blasting is caused by the roughed morphology of the surface.     

Influence of Grit Blasting on the Roughness and the Bond Strength of Detonation Sprayed Coating

The process of roughening the surfaces by grit blasting prior to coating them using thermal spray techniques is very important to obtain consistently high tensile bond strength between the coating and the substrate. The available literature on the influence of grit-blasting parameters in the case of detonation spray or HVOF coatings is quite limited. The present study aims to study the influence of grit-blasting pressure and alumina grit size on the roughening of the mild steel substrate, the resulting effect on the roughness of Cu, Al₂O₃, and WC-12Co coatings deposited by detonation spray coating and also on the tensile bond strengths of these coatings. Toward the above purpose, the velocity of the alumina grits have been experimentally measured using a high-speed imaging system and the tensile bond strength of the coatings have been experimentally obtained using the pin type test. The results from the above experiments point to the importance of not only the roughness of the grit-blasted mild steel substrate but also the roughness of the coatings subsequently deposited in determining the magnitude of the bond strength.     

Interrogations on the sub-surface strain hardening of grit blasted Ti-6Al-4V alloy

In this work we characterize the microstructural changes induced by grit blasting of the Ti6Al4V alloy and their effect on the sub-surface mechanical properties by means of micro- and ultramicro indentations techniques. It has been observed that the severe plastic deformation at the surface produces an increase in roughness. Such deformation, however, does not cause any evident hardening at the sub-surface zone, which contrast with the work hardening observed on blasted cp Ti and austenitic stainless steel 316 L. It is proposed that the different behaviour of the Ti-base alloy is related to its lower strain hardening exponent. The implications of the absence of subsurface hardening on the loss of fatigue strength observed by other authors are analysed.     

Effects of Grit Blasting and Annealing on the High-Temperature Oxidation Behavior of Austenitic and Ferritic Fe-Cr Alloys

Grit blasting (corundum) of an austenitic AISI 304 stainless steel (18Cr-8Ni) and of a low-alloy SA213 T22 ferritic steel (2.25Cr-1Mo) followed by annealing in argon resulted in enhanced outward diffusion of Cr, Mn, and Fe. Whereas 3 bar of blasting pressure allowed to grow more Cr₂O₃ and Mn _x Cr_3?x O₄ spinel-rich scales, higher pressures gave rise to Fe₂O₃-enriched layers and were therefore disregarded. The effect of annealing pre-oxidation treatment on the isothermal oxidation resistance was subsequently evaluated for 48 h for both steels and the results were compared with their polished counterparts. The change of oxidation kinetics of the pre-oxidized 18Cr-8Ni samples at 850 °C was ascribed to the growth of a duplex Cr₂O₃/Mn _x Cr_3?x O₄ scale that remained adherent to the substrate. Such a positive effect was less marked when considering the oxidation kinetics of the 2.25Cr-1Mo steel but a more compact and thinner Fe _x Cr_3?x O₄ subscale grew at 650 °C compared to that of the polished samples. It appeared that the beneficial effect is very sensitive to the experimental blasting conditions. The input of Raman micro-spectroscopy was shown to be of ground importance in the precise identification of multiple oxide phases grown under the different conditions investigated in this study.     

Erosion testing and surface preparation using abrasive water-jetting

Erosion testing and surface preparation are studied using a 3-axis Computer Numerical Control (CNC) abrasive water jetting (AWJ) apparatus. The effects of erosion time t, impingement angle α and pressure p on the erosion rate E, average surface roughness R _a, and surface hardness Rockwell C Hardness (HRC) were investigated in detail. Compared with conventional grit blasting, AWJ can reduce grit embedment in the target material due to the action of the high-pressure water. AWJ also has the advantage of generating a higher average surface roughness R _a over water jetting (WJ) due to the action of abrasive particles. In addition, AWJ increases the surface hardness HRC of the substrate material. The obtained higher degree of average surface roughness is helpful for improving the bonding strength between the coating and the substrate material. The erosion testing and the surface preparation are numerically controlled by a 3-axis CNC system; therefore precise and detailed results for various operating parameters can be obtained.     

Thermal cyclic behavior of air plasma sprayed thermal barrier coatings sprayed on stainless steel substrates

Thermal barrier coatings (TBCs) were deposited by an Air Plasma Spraying (APS) technique. The coating comprised of 93 wt.% ZrO₂ and 7 wt.% Y₂O₃ (YSZ); CoNiCrAlY bond coat; and AISI 316L stainless steels substrate. Thermal cyclic lives of the TBC were determined as a function of bond coat surface roughness, thickness of the coating and the final deposition temperature. Two types of thermal shock tests were performed over the specimens, firstly holding of specimens at 1020 °C for 5 min and then water quenching. The other test consisted of holding of specimens at the same temperature for 4 min and then forced air quenching. In both of the cases the samples were directly pushed into the furnace at 1020 °C. It was observed that the final deposition temperature has great impact over the thermal shock life. The results were more prominent in forced air quenching tests, where the lives of the TBCs were observed more than 500 cycles (at 10% spalling). It was noticed that with increase of TBC's thickness the thermal shock life of the specimens significantly decreased. Further, the bond coat surface roughness varied by employing intermediate grit blasting just after the bond coat spray. It was observed that with decrease in bond coat roughness, the thermal shock life decreased slightly. The results are discussed in terms of residual stresses, determined by hole drill method.     

Performance Optimization of Cold Rolled Type 316L Stainless Steel by Sand Blasting and Surface Linishing Treatment

Sand blasting followed by a surface linishing treatment was applied to optimize the near-surface microstructure of cold rolled type 316L stainless steel. The introduction of cold rolling led to the formation of α-martensite. Specimens with large thickness reductions (40, 53%) were more susceptible to localized corrosion. The application of sand blasting produced a near-surface deformation layer containing compressive residual stresses with significantly increased surface roughness, resulting in reduced corrosion resistance. The most resistant microstructure was obtained with the application of a final linishing treatment after sand blasting. This treatment produced microstructures with compressive near-surface residual stresses, reduced surface roughness, and increased resistance to localized corrosion.