速度

Ž .Thin Solid Films 385 2001 132�141 Role of condensates and adsorbates on substrate surface on fragmentation of impinging molten droplets during thermal spray Xiangyang Jiang� , Yuepeng Wan, Herbert Herman, Sanjay Sampath Center for Thermal Spray Research, Department of Materials Science and Engineering, Uni�ersity of New York at Stony Brook, Stony Brook, NY 11794-2275, USA Received 16 January 2000; received in revised form 30 November 2000; accepted 14 December 2000 Abstract We propose that the presence of condensates�adsorbates on low temperature substrate surfaces may be a significant factor responsible for splat fragmentation of impacting molten droplets. Vaporization and rapid expansion of condensates�adsorbates upon molten droplet impact cause instability of the spreading droplet. Plasma spraying experiments, using radio frequency induction processing of ZrO , were designed to test this hypothesis. In order to obtain different levels of condensates�ad-2 Ž .sorbates, steel substrates were heated in a vacuum chamber at 250 torr and allowed to cool under vacuum for different periods Ž .of time, ranging from 1 to 62 h before splat deposition. For comparison, splats were also produced on ambient 25�C as well as Ž .on heated substrates 500�C . It was found that splat morphology changed from highly fragmented to a contiguous, disk-like shape with a decreased level of surface condensates�adsorbates, although the substrate temperature was maintained at ambient temperature. � 2001 Published by Elsevier Science B.V. All rights reserved. Keywords: Condensates; Adsorbates; Molten drops; Thermal spray; Fragmentation 1. Introduction Fragmentation of molten droplets is a commonly observed phenomenon in many metallurgical processes, which involve the impact of molten droplets on a substrate or pre-deposited materials, such as splat quenching, thermal spray, spray forming, microcasting, � �etc. 1,2 . A fundamental understanding of fragmenta- tion mechanisms is not only of scientific interest, but has technical consequences as well. It is commonly believed that splat fragmentation is generally detrimen- � �tal to coating quality 3�6 , affecting not only the splat � �solidification rate 3 , but also the phase selection, deposit microstructure development and deposition efficiency. It is also found that cracks�voids form at �Corresponding author. Tel.: �1-309-578-6624; fax: �1-309-578- 2953. Ž .E-mail address: jiang xiangyang@cat.com X. Jiang .� ZrO splat and substrate interfaces in the presence of2 � �fragmented splats 4 . It is known that improved me- chanical and physical properties of deposits are achieved when splat fragmentation is avoided in Mo � �and ZrO systems 5,6 . A large number of studies have2 shown that contiguous morphology of splats is benefi- � �cial to the overall quality of coatings 3�5 . 1.1. Sur�ey of the fragmentation mechanism for molten droplets A suitable mechanism to explain the fragmentation of impacting droplets has not been developed. The fragmentation of non-thermal spray particles, such as rain or fuel droplets which do not experience a phase change during deposition, has received considerable attention in the field of fluid dynamics, where fragmen- tation is generally attributed to fluid flow instability, � �such as Rayleigh�Taylor or Weber instabilities 7�9 . 0040-6090�01�$ - see front matter � 2001 Published by Elsevier Science B.V. All rights reserved. Ž .PII: S 0 0 4 0 - 6 0 9 0 0 1 0 0 7 6 9 - 6 ( )X. Jiang et al.�Thin Solid Films 385 2001 132�141 133 With an increase of the Reynolds or Weber numbers, � �the fragmentation tendency increases 7�9 . However, in the case of thermal spray droplets, such as molten metal and ceramic droplets, fragmentation is pre- sumably still governed by these fluid dynamic parame- ters, but in a much more complex way due to the strong thermal interactions between the droplet and the subs- trate. It was found as early as the 1920s that substrate temperature has a strong effect on splat morphology. With an increase in substrate temperature above am- bient, splat morphology changes gradually from highly � �fragmented to contiguous 10 . This phenomenon has � �been widely confirmed 11�13 . Droplet materials in- clude ZrO , Al O , Al, Ni, Cr, Cu, Mo, etc. and2 2 3 substrate materials such as ceramics, glass and steel have been used. There is an effective substrate ‘transi- tion temperature’, below which splat fragmentation oc- curs and above which the splats are contiguous and generally well formed. The reported transition temper- atures for different droplet�substrate combinations fall into a relatively narrow range. Fukumoto et al. suggests there is a critical fragmentation�non-fragmentation transition temperature for each particular droplet�sub- � �strate combination 12 . The substrate temperature effect remains an enigma, with the controlling mechanism still unclear. Most ex- planations attribute this phenomenon to the solidifica- � �tion of the impacting droplet 12,14,15 . Differences among the theories depend only on the details of the solidification effects. Fukumoto et al. suggests that fragmentation results from rapid solidification at low substrate temperatures, in addition to high droplet � �surface tension 12 . They further argue that the initial solid layer which forms just after the impingement � �causes fragmentation through melt jetting 14 . It has also been postulated that higher substrate tempera- tures enable the melt to be maintained above a ‘hyper- cooling temperature’, avoiding the local solidification and, therefore, the creation of perturbations during � �spreading 15 . Other explanations involve wettability theory, in that improved wettability for a higher substrate temperature may be responsible for the splat morphology change � �14,16 . Severe oxidation of the substrate surface is suspected as a major contributor to fragmentation. Splat morphology changes from contiguous to frag- mented when the steel substrates are heated for ex- � �tended periods or at higher temperatures 16,17 . 1.2. Problems with the a�ailable fragmentation theories for molten droplets None of the hypotheses mentioned above can be used to explain all of the reported experimental results for different droplet-substrate systems. In our view, it is unrealistic to apply solidification arguments to systems as diverse as Mo-on-Mo and ZrO -on-ZrO , which2 2 differ in solidification rate by several orders of magni- tude, i.e. a 100�C or 200�C rise in substrate tempera- ture cannot be expected to significantly change the solidification behavior of a droplet with a melting point above 2500�C. Also, in some cases, solidification is the arresting factor in droplet spreading. Thus, splats can- not remain molten before the spreading is completed � �18,19 . Substrate oxidation can not explain the fragmenta- tion for low substrate temperatures, the trend remain- ing the same when splats are produced on glass or � �oxide substrates 16,18 . The reported transition occurs at approximately 100�400�C without an obvious depen- �dence on droplet�substrate combination 3,5,6,- �10,12,14 . These facts strongly suggest that some factor independent of substrate and droplet materials are responsible for the morphology transition. 1.3. Proposed hypothesis We propose a material-independent mechanism, which is consistent with most experimental observa- Ž .tions, adsorbed gas�condensation AGC on the sub- strate surface. During previous experiments, we observed no deposition of Mo droplets on glass subs- trates cleaned with alcohol, but found instead that Mo deposits well on untreated substrates. Sampath and Herman reported more contiguous Ni splats formed in a reduced pressure chamber than at atmospheric pres- � �sure 20 . Furthermore, additional experimental evi- dence has accumulated, including the substrate tem- perature effect, which seems to support this concept. Raising the substrate temperature is expected to effec- tively eliminate the AGC. It is known that water and other substances can be adsorbed on clean solid sur- faces. Desorption tends to occur when the temperature rises. Adsorption�desorption kinetics and reaction di- rection is a function of temperature, adsorbate species and solid surface structure and surface energy. Evap- oration of the AGC upon the impact of hot droplet may cause the fragmentation. A literature survey on the effects of AGC on droplet deposition has yielded only one relevant paper, Li et al. address the effects on the splat morphology of organic � �films brushed on a substrate surface 21 . They observed a splat morphology transition upon the removing of the organic film through heating. The purpose of the present study was to find clear experimental evidence that confirms the role of AGC on droplet fragmentation. 2. Experimental design Experiments were designed to prepare splats on sub- strates with different AGC levels, while keeping all the ( )X. Jiang et al.�Thin Solid Films 385 2001 132�141134 Ž . Ž .Fig. 1. Chamber pressure a and temperature b experienced by substrates before splats preparation. Gradual chamber pressure increases due to leaking is recorded after the main vacuum pump is shut down. The pressure for run 4, 3 reached approximately 8 and 10 torr, respectively, before the refilling of argon and it reached 50 torr for run 2. Pre-heating and spraying was at a pressure of 250 torr. Run 1: no pre-heating; run 2,3 and 4: Pre-heating, cooling down and spray after 62, 17 and 1.2 h, respectively; run 5: pre-heating and spraying. other conditions constant. By comparing splat mor- phology, the role of AGC can be inferred. Based on Ž .this approach, low pressure radio frequency RF plasma spraying was used to prepare splats in a protec- tive inert gas environment. Polished steel substrates Ž .25�50�3 mm were placed in the chamber, which was then evacuated to less than 0.1 torr. Following this step, the chamber was refilled with pure argon to 250 torr, which is necessary for the RF plasma to operate. For each run, the substrate was heated with the plasma torch for 10 s to approximately 500�C, then the cham- ber was evacuated to less than 0.1 torr and the subs- trate was cooled down for a given period of time. The chamber was then refilled to 250 torr and molten droplets were deposited on each substrate. For compar- ison, one splat specimen was prepared on an unheated substrate and another one was prepared immediately after the substrate was heated. The substrates were polished to a roughness of R �0.05 �m and cleaneda with acetone. The pre-heating and cooling histories of the substrates are shown in Fig. 1. Substrate tempera- tures are estimated with a one-dimensional heat con- duction model to be 25, 25 and 50�C for runs 2 through 4, respectively. Splats were deposited with a TAFA Model 66 Radio Frequency Plasma system. The experimental setup is shown in Fig. 2 and the spraying conditions are listed in Table 1. Ž . ŽNorton �235 partially stabilized ZrO PSZ 5.12 .wt.% calcia powder with a mean particle size of 63 �m was used in this investigation. Optical microscopy and scanning electronic micros- Ž .copy SEM were used to observe splat morphology. Secondary electron, back-scattering and specimen cur- rent images taken of the as-sprayed splats were used to evaluate the actual splat contact on the substrate. During SEM observations, an electrostatic charge builds up in the insulating PSZ splat, in the good contact area, the charge leaks to the substrate and forms the specimen current signal, therefore, bright areas in the specimen current image represent ‘good’ splat�sub- � �strate contact areas 22 . 3. Results Table 2 summarizes the substrate heating histories and the resulted substrate surface AGC levels, splat morphologies, etc. The results shown in Table 2 are discussed in the following sections. It is clear that splat morphology changes from highly fragmented to con- tiguous with a decrease in AGC level on the substrate surface, along with an increase in actual contact area and crack density. 3.1. Morphologies of splats prepared under different conditions It is observed that the splats prepared on the un- Ž .heated substrate sample 1, shown in Fig. 3a have highly fragmented shapes which generated a large quantity of debris. There are many small holes through each splat with pits and humps located throughout the splats. Table 1 Spraying parameter Plasma gas and Carrier gas Current Voltage Torch�substrate Substrate Substrate Ž . Ž .flow rate and flow rate A V distance heating time rotating rate Ž . Ž . Ž . Ž . Ž .slm slm cm S rev.�min Ar100�He 140 He 4 10 5, 700 26 10 120 ( )X. Jiang et al.�Thin Solid Films 385 2001 132�141 135 Fig. 2. Schematic diagram of the experimental setup for splat prepa- Ž .ration not to scale, vacuum chamber not shown . Splats prepared on substrates that were pre-heated and then cooled down to ambient temperature in vac- uum, show very different morphologies. On a substrate maintained under vacuum for 62 h, most splats pre- Ž .pared after the pre-heating sample 2 show a much more uniform morphology with clear flow patterns and long projections along the periphery of the splats, Fig. 3b displays highly fragmented, irregular shaped splats and debris. This suggests that the fragmentation ten- dency is still very high, but in a much more regular and uniform way. ŽOn the substrates allowed to cool for 17 h sample .3 , the splats have essentially disk-like shapes with a distinguished rim, humps can be found in the central Ž . Žarea of some splats Fig. 3c . Only a few splats less .than 10% with less contiguous shapes were observed. Cracks can be seen in some splats. Ž .On the substrates allowed to cool for 1 h sample 4 , Žthe splats have very contiguous, disk-like shapes Fig. .3d . A crack grid pattern can be discerned in the central part of most splats, but not on the rim. In some smaller splats, crack grids are well developed through- out the entire splat, including the rim. Splats prepared immediately after substrate heating Ž .sample 5 show a contiguous, disk-like shape with Ž .well-developed crack grids on all splats Fig. 3e . Com- pared with the splats in sample 6, the crack grid is much finer and exists throughout the surface of the entire splat. 3.2. Effecti�e contact�adhesion of splats on the substrate It is noted that the splat-substrate adhesion is greatly enhanced when the substrate is kept in vacuum for shorter periods of time between pre-heating and splat deposition. For the splats produced on unheated subs- Ž .trates sample no. 1 , most splats are loosely attached to the substrate and many splats are lost while handling the substrate during optical observation, with only some debris remaining. Splats produced 62 h after substrate pre-heating shows some improvement. With the short- ening of the dwell-time in the chamber, most splats remain on the substrate. Splats deposited on heated Ž .substrates sample 5 adhere well. It has been shown that while splats prepared on cold substrates are loosely bonded, the splats produced on hot substrates can � �sustain indentation without delaminating 23 . Deposits prepared on hot substrates also show greater adhesion over those prepared on cold sub- � �strates 16 . This is supported by the observation of crack patterns in the splats. Cracking is a mean of stress relief caused by the constraint of the splat to the substrate during quenching and cooling. The appear- ance of cracks indicates that the adhesion has im- proved. Adhesion is greatly enhanced when the time interval between pre-heating and spraying is shortened. SEM current images of the splats reveal dramatic differences in the contact ratio of splats on each of the different substrates. Splats produced on unheated subs- Table 2 Summary of substrate temperature history, surface AGC level and splat morphology Ž .Sample run no. 1 2 3 4 5 Substrate temperature Ambient Ambient�500�C ambient�500�C Ambient�500�C Ambient�500�C history �ambient �ambient �50�C Cooling time No heating 62 17 1.2 0 Ž . Ž .after heating h infinite AGC level Very high Medium Moderate Low Very low Splat Highly Fragmented Mostly Contiguous Highly morphology fragmented contiguous contiguous Contact area Very Low Medium High Very high low�none Crack density No No Medium�low High Very high ( )X. Jiang et al.�Thin Solid Films 385 2001 132�141136 Fig. 3. Morphology of typical splat prepared on pre-treated subs- trates substrate condition. Ž .trates sample 1, Fig. 4 a2 show virtually no contact with the substrate, splats from sample 2 show an in- Ž .creased contact area at the center Fig. 4 b2 , splats from sample 3 show even larger ‘good’ contact areas Ž .Fig. 4 c3 . Splats from samples 4 and 5 show much larger contact area, but the contact pattern changes from a large patch to an area of mixed good�poor contact patches. This change is accompanied by in- creased crack densities and reduced splat lift-over at the rim. In all cases, the contact at the splat rims is poor. The back-scattering electron images of the splats reveal that the dark areas in the secondary electron images are not indications of holes, but an artifact due to the electrostatic charging effect. The contact area pattern change is probably due to splat cracking. Upon impact and spreading of the droplet, the contact at the center is good and continu- ous. This contact exerts a constraint on the splat and causes cracking. Splat segments lift near the crack edge, causing the splats to detach from the substrate and form poor contact areas. 4. Discussion 4.1. Analysis of the substrate surface conditions Before we can attribute the above experimental re- sults to AGC on the substrate surface, it is necessary to first analyze the surface condition, such as surface oxidation, roughness, temperature and the levels of substance condensates�adsorbed for different experi- mental cases. 4.1.1. Surface oxidation� roughness A thin oxide film is always present on steel exposed to atmosphere. No significant further oxidation is ex- pected because substrates were maintained in vacuum during pre-heating and subsequent cooling. Therefore, a change in the wettability of a molten droplet on the steel substrate due to the surface oxidation after sub- strate pre-heating, is unlikely. The substrate surface was also inspected with optical microscopy before and after depositing the splats for possible changes in surface roughness. No change was observed. 4.1.2. Substrate temperature Substrate surface temperatures dropped to ambient after 1 h of cooling as shown in Fig. 1 for samples 2, 3 and 4. This simple one-dimensional heat conduction model gives a conservative result by neglecting heat loss through radiation and convection. Substrate tem- perature differences, therefore, are not the reason for the observed splat morphology change under these conditions. 4.1.3. Adsorption and condensation on the substrate surface It can be inferred from the following analysis that the different levels of AGC were achieved in different Ž .experimental runs i.e. samples in this investigation. It is well established that clean surfaces energetically attract foreign species, which results in adsorption and condensation of molecules. Condensed volatiles vapor- ize when the vapor pressure is lower than the satura- tion vapor pressure at a given temperature. The evap- oration rate increases with temperature and also with a decrease in partial pressure of the condensed species. The most common condensate is H O, which has a2 vapor pressure of 17 mmHg above that of the liquid at room temperature. In general, the equilibrium volume of an adsorbate sharply decreases with an increase in temperature and a decrease in pressure. Multi-layer adsorption can take place and the adsorbed volume can be very high and possibly go to infinity as t