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ZnS/Diamond Composite Coatings for Infrared Transmission Applications Formed by the Aerosol Deposition Method

The deposition of nano-crystalline ZnS/diamond composite protective coatings on silicon, sapphire, and ZnS substrates, as a preliminary step to coating infrared transparent ZnS substrates from powder mixtures by the aerosol deposition method is presented. Advantages of the aerosol deposition method include the ability to form dense, nanocrystalline films up to hundreds of microns thick at room temperature and at a high deposition rate on a variety of substrates. Deposition is achieved by creating a pressure gradient that accelerates micrometer-scale particles in an aerosol to high velocity. Upon impact with the target substrate the particles fracture and embed. Continued deposition forms the thick compacted film. Deposition from an aerosolized mixture of ZnS and diamond powders onto all targets results in linear trend from apparent sputter erosion of the substrate at 100% diamond to formation of a film with increasing fractions of ZnS. The crossover from abrasion to film formation on sapphire occurs above about 50% ZnS and a mixture of 90% ZnS and 10% diamond forms a well-adhered film of about 0.7 \mu m thickness at a rate of 0.14 \mu m/min. Resulting films are characterized by scanning electron microscopy, profilometry, infrared transmission spectroscopy, and x-ray photoemission spectroscopy. These initial films mark progress toward the future goal of coating ZnS substrates for abrasion resistance.


INTRODUCTION
Many infrared (IR) sensor applications require optics with good tranparency in the 3 -5 and 8 -14 µm wavelength range and good mechanical performance against harsh environmental conditions such as rain and sand impact and thermal shock. 9 Of the known materials the so-called "two-color" or "multispectral" material ZnS is a good candidate due to strong transmittance in this desired range even at high operating temperature due to its large bandgap of 3.6 eV. Unfortunately, its poor mechanical properties require a durable protective coating. Diamond is a strong candidate as a protective coating material for several reasons. It has good transparency in the long-wave IR and has mechanical properties orders of magnitude better than other optical materials. It has twice the mechanical strength of ZnS and over 70 times its thermal conductivity. A measure of thermal shock at mild heating, the Hasselman figure of merit, for diamond is 200 times better than ZnS. For these reasons there has been much focus on depositing diamond and diamond-like protective coatings onto ZnS, but processes such as chemical vapor deposition require high temperature or other operating environments inimical to the ZnS structure. While there have been some advancements in coating ZnS with a hard diamond layer by plasma laser discharge 7,8 and diamond composites by filtered cathode vacuum arc 19 there is to date no current technique that meets the current application needs. An improved deposition technique would deposit diamond and/or diamond-containing composites at low temperature, build up a several micron-thick dense film that is well adhered to the substrate, offer improved abrasion resistance, and cover a large surface area.
For this reason the novel aerosol deposition method (ADM) has been employed to potentially solve this long-standing problem.
The ADM is a film fabrication technique that utilizes an impact solidification phenomenon of fine particles that was invented in 1997 by Dr. Jun Akedo. The ADM has demonstrated the most success of any technique for depositing ceramic and metallic compounds 1,4,11,18 such as, α- onto a substrate, such as Cu, Al, glass, stainless steel, Si, SiO 2 to form thick (> 100 µm) dense (95% bulk density) films at room temperature. Recently, ADM has been used to deposit mixtures of Al 2 O 3 and nano-diamond onto glass for anti-scratch and smudge resistance. 15 There are four main components of the ADM system: 1) an aerosol chamber (AC); 2) a deposition chamber (DC); 3) carrier gas source; and 4) a vacuum pump as shown in Figure 1. In this process a chamber containing an aerosol of fine (0.1 -10 µm) particles is pressurized with nitrogen, helium, or oxygen while a chamber containing a target substrate is pumped to < 1 Torr. The pressure differential ∆P (∼ kinetic energy) accelerates the aerosolized particles through a nozzle toward the substrate. Previous reports of empirically measured velocities are 150 -550 m/s for Pb(Zr 0.52 ,Ti 0.48 )O 3 (PZT) 2 and 650 m/s for ultrafine silver particles. 14 Figure 2 illustrates the currently accepted deposition mechanism. The film begins to form when incident particles impact, fracture, and embed into the substrate. This impact causes indentation and abrasion of the target area giving rise to an increased surface area which facilitates formation of a well-adhered anchor layer comprised of 10 -100 nm sized particles. Subsequent impacts serve to compact the underlying film and bonds the crystallites. Since only fracturing occurs, the fabricated film has the same crystalline structure as the raw powder. The structure of the film is a comprised of densely packed crystalline nanoparticles held together by what is thought to be close-range mechanical and chemical interactions mediated by fracture, and/or plastic deformation of the particles. 2 These films are more than 95% of the density of the bulk material. 1 Several reports claim that the deposited particles retain the crystalline microstructure, although signs of strain in the deposited particles can also occur. 2,11,12,16,18 Finite element simulations suggest that the pressure and temperature at the impact site is not sufficient to cause melting, 2, 3 but to date a more thorough investigation of the mechanisms of adhesion in this process has not been reported.
The ability of the ADM to form dense thick films at room temperture over a broad range of material systems makes this novel method a potential solution for coating diamond or diamond-containing composite films onto ZnS substrates. We present the data for deposition onto ZnS, silicon, and sapphire substrates. While the immediate motivation is to coat ZnS, by comparing the deposition results of the three system a broader understanding of the deposition mechanism may be ascertained.

MATERIALS & METHODS
The ADM used in this work is a custom built system at the U.S. Naval Research Laboratory, Washington, D.C. (NRL).    Deposition targets were p-type, boron-doped <100> single-side polished silicon wafers 380 µm in thickness, aplane double-side polished sapphire wafers 320 µm in thickness, and ground ZnS wafers 508 µm in thickness) . The wafers were diced to 1.5 × 1.5 cm squares and masked with cellophane tape. The deposition target area was exposed by cutting out a 2 × 10 mm section using a stencil and razorblade (see inset of 3. RESULTS highlights the damage of diamond particle impact extending about 0.5 µm into the substrate. The extent of the surface damage evident in these figures is confirmed by profilometry (see Figure 6 and accompanying text). These images are representative of similar results when using longer deposition times, lower or higher ∆P (kinetic energy), and diamond particles of sizes from < 0.25 to 10 µm in the NRL ADM system. These initial depositions motivate introducing a "binder" material that could facilitate adhesion to the substrate and help absorb the initial impact of the diamond. In addition, a suitable material with a better thermal match to the ZnS substrate might decrease film delamination from thermal cycling. An obvious material to choose is ZnS powder.  Figure 5 shows a delaminated portion of a film deposited with a 70% mixture that has broken away from the edge. A well-compacted conglomerate of larger diamond shards and the indistinguishable ZnS particles illustrate that the film is well compacted and comprised of both ZnS and diamond even at 70% ZnS content.       background subtracted and dotted fit line overlayed. As the main plot indicates, the total count increases with increased Zn percentage as one might expect. Note the 0% mixture at the origin. The sudden drop at 100% could be due to poor coverage due to poor adhesion. At percentages below 60% where a negative step height might suggest a purely abrasive mechanism there remains a fairly strong Zn signal. The presence of Zn, ergo ZnS, indicates that other mechanisms might be at work than pure abrasion of the substrate.

DISCUSSION AND CONCLUSION
Deposition with pure diamond produces a substrate that is abraded, surface roughened, and sparsely embedded with the particles. While diamond-on-diamond adhesion in this process would be desirable, the interaction may be very elastic. The bounce kinetic energy KE b ∝ (1 − e 2 )/e 2 is the kinetic energy required for a bounce to occur between two interacting objects. 6,10 Here e is the coefficient of restitution as determined by the ratio of the difference in the incoming and outgoing velocities. For an impact between two nearly perfectly elastic objects such as diamond particles e ∼ 1 resulting in diamond bounce and/or fracture, but not adhesion. Serendipitously, the impact and damage resulting from the diamond may actually enhance deposition by softer materials, such as ZnS, by increasing the surface area and forming regions where particles can impact inelastically. This is affirmed by the poor film adhesion using 100% ZnS, which is easily brushed off. Addition of ZnS into the diamond powder may provide a more inelastic impact, effectively raising KE b . With increasing ZnS percentage less diamond is likely present in the aerosol, but the total amount of diamond adhering to the substrate is increased since there is more overall material deposited.
Furthermore, diamond even at higher percentages of ZnS is clearly interacting with the substrate and producing a rough surface which is likely continuing to facilitate the adhesion of both ZnS and diamond (see Figure 4 (b) and (d)). Inherent in this process are the mechanical properties materials. The substrate plasticity, evident in the SEM images, may explain why film growth occurs on silicon and sapphire, but not ZnS. Local plasticity has been found to occur in both sapphire and silicon at pressures of 10s of GPa, which coincide with particle impact pressures during ADM. 2 ZnS, which is about 5 times softer than silicon may not provide adequate plasticity at these pressure to withstand the particle bombardment. This motivates several avenues of study. Two such studies might be on surface treatment of the ZnS substrate and decreasing the particle impact velocity.
The combined data from the SEM, profilomentry profiles, FTIR spectra, and XPS spectra suggest that there may be another mechanism involved in the deposition process than simply abrasion of the substrate. SEM images show that the local surface structure across all mixtures is similar. Measurements of rms roughness confirm that only a slight increase in roughness occurs in going from 0% -100% (data not shown). XPS data show a clear Zn presence on the surface at all but the 0% mixture and transmittance variance suggests a change in the bulk material. A possible mechanism could be compression of the substrate surface due to high pressure bombardment of particles. Reports of nano-indentation of less than 10 GPa onto silicon have shown a structural phase change which results in a 21% volume collapse. Rapid compression and release could result in a meta-stable structural phase with a substantially smaller volume accounting for the apparent abrasion and existance of particles adhered to the region.
There are several strategic avenues to pursue to address many of the questions still outstanding in this work. One such study would be to develop a thin impact absorbing film coating on ZnS to increase KE b . The energy absorbed by the film during impact may help decrease abrasion and micro-fracturing in the substrate. Additionally, lower energy impact using smaller particles and lower velocities might improve substrate abrasion. Another approach might be to incorporate the properties of ZnS and diamond together as a core/shell particle. An advantage of this approach would be to lower the impact of each diamond particle and facilitate adhesion both with the substrate and other core/shell particles. Abrasion and wear testing is an obvious neccesity in future work. Comparing the as-deposited films with those sintered in an oxygen-free environment or by hot isostatic pressing may improve film density and abrasion resistance. In conclusion, we utilized a newly constructed system to perform the aerosol deposition method of impacting diamond and mixtures of ZnS/diamond onto silicon, sapphire, and ZnS substrates to better understand the deposition mechanism with the goal of achieving deposition onto ZnS. The results of this work suggest that film formation depends on particle velocity (∆P), substrate mechanical properties, and ZnS/diamond percentage. We achieved a maximum film thickness of 0.9 (0.7) µm onto silicon (sapphire) with a 90% ZnS mixture likely facilitated by the local mechanical structure of the silicon and sapphire and sustained substrate deformation by the diamond. All the targeted deposition areas showed very high transmittance in the 4 -7 µm wavelength range with a constant surface roughness. Future work to employ a suface coating on ZnS, using smaller particles, decreasing the impact velocity, and synthesizing a core/shell particle may provide beneficial to improving abrasion and producing thicker films. A future film study will also include sintering and abrasion resistance testing.