Surface modification of lotus-type porous copper by aluminization
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This paper explores the feasibility to aluminize lotus-type porous copper to prolong the service life of lotus-type porous copper in applications. To achieve this goal, smaller particles of α-Al2O3, Al and NH4Cl were employed for aluminization process considering the difficulty arising from shielding effect by pore walls. The structure of the aluminized coating including thickness, elements profile and phase composition was characterized. It is found that the deposition of aluminized coating on pore walls of the lotus-type porous copper is feasible. It is indicated that the pore size has to be more than 300 µm in order to obtain a decent aluminized coating on pore walls of the lotus-type porous copper under the present experimental conditions. The coating is consisted of an alloy layer (first α-Cu, then a mixture of Cu3Al and Cu9Al4) and a re-solidified layer (α-Al2O3 particles incorporated in Al substrate). Furthermore, Vickers hardness, sliding property and wear resistance were evaluated. The hardness is enhanced by a factor of three, which is distributed from 105 to 302 HV0.01 from copper substrate to the coating surface. It is also proved that the aluminized coating can significantly reduce the wear rate of the lotus-type porous copper by shifting the wear mechanism from adhesive mode to plowing mode by the improved hardness.
KeywordsLotus-type porous copper Surface modification Aluminization Wear resistance
Lotus-type porous metals with elongated directional pores fabricated by mold casting technique have been studied for decades for their superior physical and mechanical properties. These metals can be tailored to achieve controllable porosity, pore size, pore growth direction and uniform distribution of pores, as well as high specific strength and stiffness . Therefore, this porous metal finds many applications to serve as both functional and structural materials, including heat sink , vibration–damping material , lubricating material , tissue material [2, 13], and so on. The porous metal with uniform pore size and porosity has been achieved on the metals with high thermal conductivity such as copper. However, in the case of metals especially alloys with low thermal conductivity, porous metals with uniform pore size and porosity could not be fabricated due to poor solidification velocity of the melt. Although, new methods such as continuous zone melting technique and continuous casting technique were developed by Nakajima  to resolve these issues. Yet, pore size and porosity distribution are still not as uniform as pure metals with high thermal conductivity by the mold casting technique.
Copper is the most popular material for lotus-type structure with some beneficial properties like high thermal conductivity and high strength, which make it favorable and promising for filtration and thermal industry [7, 8, 19, 24]. However, this metal has relatively lower hardness and is prone to adhesive and abrasive wear when used for filtration and separation purpose. In addition, it has some unfavorable characteristics like surface oxidation  and corrosion. One feasible way to improve the wear resistance of Cu is to incorporate a hard metal in the Cu matrix so as to increase the hardness . However, the presence of some additional doped elements in solid solution may reduce the electrical conductivity of the copper alloys . Surface modification offers another route for improvement on those properties  , as wear and corrosion in metals are mainly controlled by the surface properties of the material . One widely used approach to tailor and improve properties of metal is to coat it with another material that exhibits superior properties [1, 2, 4, 11, 12, 14]. In the case of porous metals, the pore structure presents certain degree of difficulty to modify the surface due to the shielding effect from pore walls. Liu et al.  used the micro-arc oxidation method to deposit Al2O3 coating on open-cell Al foam. Their results showed that the coating was composed of a dense internal layer and a porous external layer. This particular structure improved the corrosion resistance of the open-cell Al foam. Wei et al.  aluminized porous stainless steel to achieve an intermediate layer which further facilitated the deposition of palladium membranes.
In the case of lotus-type porous metals, many researchers have put their efforts to mitigate the problem with the purpose to improve the desired properties. Nakajima et al.  deposited zinc on lotus-type porous copper by vaporization and diffusing the zinc into copper matrix via annealing to form a brass layer. Li et al.  deposited zinc layers on lotus-type porous copper by electroless plating and heat treated the samples to form a brass layer too. It can be predicted that the corrosion resistance of the coated lotus-type porous copper would be improved in both Nakajima’s and Li’s works. Du et al.  deposited nickel on the lotus-type porous copper by electroplating method which resulted in an improvement on both compression strength and absorbing energy capacity for the porous copper. Ikeda and Nakajima  deposited a titanium film by vapor deposition on lotus-type porous stainless steel SUS304L for a better biocompatibility. These studies have proven that the surface of lotus-type porous metals can be modified by different surface treatments to serve different purposes. However, all the coatings mentioned above are at the scale of 10 µm in thickness. In real working environment, coatings with thickness at the scale of 100 µm are more favorable for providing wear, oxidation and corrosion resistance.
It is the gaseous AlCl3 that reacts with Cu to deposit Al on Cu substrate. So, it may be very well applicable to lotus-type porous copper as the process is essentially a high-temperature chemical vapor deposition (CVD) process whereby AlCl3 vapors can gain access into the inner walls of the porous copper. In addition, little impact on the pore size was found on the porous stainless steel with smaller pores by the same process in Wei et al.’s work . To date, most of the reports on intermetallic compound coatings on copper surface are mainly focused on oxidation resistance improvement . To the best of our knowledge, little information is available on surface modification of lotus-type porous metals via aluminization process.
In this work, possibility of applying aluminization process on lotus-type porous structure (metals) has been explored. The process was carried out on a lotus-type porous copper with an attention on pore walls. The structure of intermetallic coating on the pore walls of the porous copper was characterized, including cross-section, thickness, composition and phase structures. Furthermore, mechanical properties of the aluminized lotus-type porous copper including Vickers hardness, sliding property and wear rate were evaluated. Finally, wear mechanism was discussed for the aluminized lotus-type porous copper.
2 Experimental procedures
2.1 Aluminization process
With the purpose to aluminize the pore walls inside the pores, the powder used in this process is relatively small. The Al2O3 particles are irregular cubic in shape and 10 µm in size, and the Al particles are spherical in shape and 30 µm in diameter. The powders were well mixed by a ball mill for 1 h for the next step. To perform the aluminization process, the specimens were sunk in the powders in a sealed glass tube filled by Ar gas in 1 atm., then heated to 800 °C at a rate of 10 °C/min with a holding time for 3 h in a muffle furnace. The specimens were fetched out after furnace cooling to less than 100 °C at its natural rate, and then cleaned with deionized water and alcohol to remove any residual powders.
The specimens were first cut into 5 mm × 5 mm × 5 mm using an electric discharge machine (DK7763, Longhao Digital-Controlled Machine Corp., China), then degreased and rinsed using the same method described above. Prior to etching in an alcohol solution with 0.2 mol/L FeCl3 and 0.2 mol/L HCl for 30 s, specimens were grinded by SiC papers from grit 400# to 1000#. The etched specimens were observed by an optical microscope (OM; MEF4A, Leica Microsystems, German). Pores with different sizes were cut along the growth direction and observed by the OM to determine the coating thickness along the pores. An X-ray diffractometer (XRD; D8 Discover, Bruker AXS, Germany) operating with Cu Kα (λ = 0.154056 nm) radiation was used to analyze the phase structure of the coating, in which the scanning was performed on the surface and different layer on the cross-section of the coating, respectively. The diffraction angle 2θ ranged from 10° to 90° by a step of 0.04°. Meanwhile, the coatings were observed by a scanning electron microscope (SEM; JSM-6301F, JEOL, Japan) on cross section, while the elements distribution of the coatings was profiled by EDS (EDS; INCA L300QI, Oxford Instruments, United Kingdom). The relationship between the pore size and the coating thickness was also measured.
2.3 Evaluation on hardness, sliding property and wear rate
3 Results and discussion
3.1 Structure of the aluminized coating
After the aluminization process, it is exciting to find that the porosity of the lotus-type porous copper decreased from 48.3 to 42.6% based on the weight gains, which is much lower than the predicted value (22.9%) considering the ratio of coating thickness (150 µm) and average pore size (575 µm). This big gap on the porosity between the experimental and the predicted values may be caused by the fact that Al diffuses into the substrate and Cu also diffuses out to the coating, which will be investigated in our further work.
3.2 Effect of pore size on aluminized coating thickness
By the images, the relationship between the pore size and the coating thickness is counted and shown in Fig. 4b. It can be found that the pore size has significant impact on the coating thickness: the coating thickness is very low when the pore diameter is under 350 µm; then, increases rapidly after the pore diameter reaches 450 µm, on both alloy layer and sintered layer; finally, remains nearly the same as that of the surface of the sample when the pore diameter is larger than 700 µm. In the case of the pores larger than 650 µm, it is interesting to find that the coating thickness starts to distribute uniformly along the pores. It is predicted that the aluminization process may be also achieved on the pores with diameter under 350 µm as well as coating uniformity when a pressurized container could be employed.
3.3 The hardness, the sliding property and the wear rate
An aluminized coating can be achieved on the lotus-type porous copper with uniform thickness along the pores. The coating is consisted of two layers: an alloy layer which is made of two phases (α-Cu and the mixture of Cu3Al and Cu9Al4) and a sintered layer where Al2O3 particles incorporated in Al substrate.
The thickness of the aluminized coating is strongly dependent on the pore size under the current experiment condition: coatings on the larger pore are thicker than those on the smaller pore. It is hard to form an aluminized coating on the pores smaller than 300 µm in diameter. The relationship between the thickness and the pore size is not linear.
The hardness is enhanced by a factor of three, which ranges from 105 to 302 HV0.01 from copper substrate to the coating surface. Due to the shift from adhesion to furrow in wear mechanism caused by the coating, the wear resistance is improved by three quarters despite of the elevated coefficient of friction for the lotus-type porous copper.
The financial support from Key state project of China (2017ZX02201001) is acknowledged.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.