Surface Modification

High-temperature, Hard and Corrosion-resistant Coatings:

A key application for our technology relating to proprietary thin film nitride-based coatings is the advantageous surface modification of different materials.  IMS has the capability to grow nitride-based coatings composed of single layer and multilayer films by ion-assisted and neutral-assisted physical vapor deposition (PVD). Useful nitride coatings – particularly cubic boron nitride, or c-BN.  Their advantage lies in the fact that nitrides not only share many useful properties of diamond films, namely; extreme hardness, high thermal conductivity, and low thermal expansion, but offer an improvement such as better resistance to oxidation and corrosion at high temperatures and significantly lower surface roughness.  Moreover, because carbon does so readily reacts with iron, limits the use of diamond in its tribological applications, where ferrous materials are involved. Nitride-based films are presently considered as very promising alternative to diamond films.  A number of compounds such as TiN and Ti-C-N show particular promise and are, in fact, in use as coatings for harsh environment.  Recent studies have indicated that there are certain chemical and mechanical advantages to the use of multilayer coatings.  Such coatings can potentially meet many requirements (for example: multifunctional character, moderate residual stresses, good adherence to metallic substrates, proper hardness to toughness ratio, and low friction coefficients) needed for a composite that would be exposed to harsh-wear conditions.

We have a capability to grow thin films of BN, CN and TiN on several substrates (semi-insulating and highly doped silicon, cubic and hexagonal silicon carbide, sapphire, borane silicate, molybdenum, gallium nitride, diamond, and stainless steel).  The deposited insulating BN thin layers are mechanically hard (Knoop hardness values of ~ 3350 kg/mm2), uniform (rms roughness ~ 15.0 Å), very smooth surface (coefficient of friction of 0.34 is slightly lower than the value measured from stoichiometric TiN, a widely-used hard coating), and porosity-free materials.  In terms of high temperature applications, BN films show good thermal stability up to 1000 oC after thermal vacuum annealing.  The combined superior smoothness with low friction and high hardness properties makes BN material a good candidate for coating applications, including corrosive and high temperature environments.

AFM topographic image of a typical BN film. RMS roughness in the order of 1.0 nm.

Infrared transmission spectrum of c-BN (80%) film deposited at 500 °C with Neutralizer Atomic Beam Source.

Micro-Column Arrays (MCA):

MCA structures created on the surfaces of different materials change the surface morphology in different ways depending on the type of material.  The most fundamental inflicted change is the enhancement of the total surface area, which can be increased up by a factor of 10 times.  Three distinct applications take advantage of such surface area enhancement: Material bonding, creation of an IR calibration source, and improvement in heat exchange across materials boundary.
 

Materials Bonding:

Drastic improvement in bonding strength between similar and dissimilar materials with surfaces bearing MCA microstructures can be achieved.  The improved bonding strength is attained with both adhesive- and brazing-based bonding modality.   Materials include conductive and refractory metals, ceramics, composites, and polymers.  For example, test data (shown below) for bonding samples of Ti alloy with reinforced ceramic exhibited an increase in the bond strength by a factor of over 50% when one surface had the MCA structures and over 90% when both were structured in comparison to bonding samples with non-structures surfaces.

 

 

Blackbody Calibration Source:

IMS has developed a novel thermal surface control based on three-dimensional micro-column arrays (MCA) that can be directly formed on the surface of virtually any solid materials, including high-temperature/refractory metals, semiconductor materials, ceramics, and even organic surfaces. Our preliminary results indicate that a metal surface with a 3D-array of micro-columns has a very low, practically negligible reflection coefficient, due to multiple reflections of the incident radiation on the micro-tips. MCA-structured surface is a perfect model of a black body since it exhibits high thermal emittance (xt) and an emissivity close to 1.0 over a wide range of the electromagnetic spectrum.

The pulsed laser ablation process used for MCA fabrication, usually results in some level of oxidation or carbonization of the MCAs, thereby passivating the surface, but chemically unaltered or specifically tailored surfaces can be also produced by placing the material under treatment into various specific gaseous or liquid ambient.

The fact that the MCAs are fabricated by modifying mostly the geometrical surface profile, means that, unlike the spray-on thermal control coatings, there will be no bleaching problems.  MCAs can be manufactured to be resistant to most environments by nitridation, carbonization and oxidation performed during the ablation process or by depositing inert materials such as Boron Nitrides.  MCAs feature-sizes can be varied, and are typically in the order of 20 microns to 120 microns.  We believe this may allow generating a wavelength-selective surface, which could either be reflective or absorptive in a very narrow bandwidth.

The MCA samples were tested for reflectance and/or absorption in the visible, near IR, and IR regions. Comparison of the MCA structured surfaces with the un-processed materials show a tremendous reduction of reflectance, corresponding to absorbances of 0.97 to 0.98 averaged over the designated spectral range.
 

Heat Exchange:

Preliminary modeling indicates that MCAs are very effective heat reducers compared to smooth metal surfaces. A 15% reduction in temperature is realized as shown in a FEM analysis of the temperature of bare titanium and titanium-MCA with a fixed heat flux. This value is expected to depend strongly on the aspect ratio of the MCA. Thermal experimental results on Alloy 321 MCA show a temperature reduction of up to 275 degrees at 30 W/cm2. This is comparable to simulation results obtained for the MCA aspect ratio of 5.443.

 

 

 

 

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