ADVANCED BONDING OF VERY DISSIMILAR MATERIALS
ADVANCED BONDING OF VERY DISSIMILAR MATERIALS
Joining processes are essential for the large scale manufacturing of components using Ti-based alloys, steels, ceramics, and composite structures. In general, joining techniques for similar metals include welding, brazing, and solid-state diffusion bonding for high temperature applications. Ambient temperature applications do not necessitate these joining processes, and joining by adhesives is sufficient in many applications. This is even more important for joining complicated structures and dissimilar materials. Tremendous advances have been made in the development of strong, easy to use, low cost, and reliable adhesives. Adhesives are everywhere: in defense, transportation, construction, packaging, appliances, electronics, product assembly, medical, and dental applications.
In adhesive and brazing bonding, the adhesive or braze transmits the load across the joint. The overall bond strength is dependent on the effective bond area between the adhesive and the material being bonded. Several techniques were examined in the past in order to improve surface area. These techniques include etching, pitting, and generation of porous surface structures. Increase in the surface area by these techniques is limited, as compared to the recently invented process of Micro-Column Array (MCA) generation by laser processing, which is the central theme of the below projects.
Adhesive Bonding of Ti to SiC Ceramic
The objective of this research project was to develop an advanced manufacturing process for bonding of dissimilar materials by employment of Micro-Column Arrays fabricated by laser ablation.
The efforts were organized into four tasks:
Task 1: Synthesis of MCA on Ti plates.
Task 2: Fabrication of Ti/Ti and Ti/SiC composite samples using epoxy bonding.
Task 3: Characterization of the treated surfaces and measurements of the bonding strength.
Task 4: Data analysis
The optimization of the MCA fabrication process was performed by wide-range variation of such laser ablation parameters as: average power, frequency, and scanning speed, and post-characterization of the micro-structured Ti samples by Scanning Electron Microscopy (SEM). The figure below is an image of laser treated (A) and untreated (B) surfaces of Ti samples:
Selection of the epoxies for the sample fabrication was based on specifications provided by the manufacturers in view of achievement of highest strength at maximal application temperatures.
Considerable increase in strength and ductility has been found in small samples. The observed value matches well with the ideal fracture strength of the epoxy. This essentially indicates that the present method of bonding can result in fracture strength of the epoxy itself, in place of a fraction of epoxy strength achievable by any currently known process.
Stress testing results on various samples are given in the table below:
|Sample No||Area, sq. mm||Thickness, mm||Maximum shear stress|
|SiC-Ti (Small samples)|
T: Laser- treated Ti
UN: Untreated surface
The expected material joint improvement was based on the increase of the specific area of the joint and modification of its surface microstructure on the submicron level, so only adhesive properties (rather than the chemical composition) of the epoxies were considered .
Surface modification of titanium by generation of micro-column arrays (MCA) using laser ablation has increased the surface area by about ten times. In addition, surface oxidation of the MCA locally increased the surface area on the modified surface itself. The oxidation also contributed to the generation of micro-pores, which possibly contribute to improvement of the adhesive bonding by providing locking sites for the epoxy.
Figure below shows the experimental setup for testing SiC/Ti samples
Surface oxidation in the present case occurred under conditions, which were not controlled. The larger size oxide particles appear to detach from metal surface, especially when the thickness of the epoxy is in the order of few tens of a micron. While the reason for the lower strength associated with lower thickness of the epoxy is not clear, it is visualized that the oxide particles on either surface might be sliding against each other during the shear test, leading to poor fracture strength.
Application of ultrasonic vibrations improve wetting of surfaces and filling of inter MCA cavities with epoxy. Application of a thin layer of epoxy, use of ultrasonic oscillations, and subsequent coating of a second layer of thick epoxy on surfaces result in improved fracture strength. This procedure developed during the project clearly demonstrated that the fracture strength of the adhesive bond reaches the strength of the adhesive itself. A plot for the sample T-T-2, for which fracture strength reached the highest value (22.8 MPa or 3420 psi) is shown below:
Ultra-Strong High-Temperature Bonding of Titanium to Ceramic Materials
The goal of this research was development of novel ultra strong high-temperature bonding of titanium (Ti) to ceramic materials, through application of advanced nano- and micro scale structures. In particular, this project will focus on improvement of the bond between a Ti attach ring and the ceramic radome of a missile.
The unique features and benefits of this technology are:
- interlocking of the braze material between micro columns
- increase in the specific surface area by more than an order of magnitude
- inherent elasticity of the micro cones providing higher resistance to shear stress
- repeated bend contours of the surface preventing hydrothermal failure.
- improved wettability due to highly developed (at the micro/nano scale) surface morphology and possibility to tailor the chemical composition of the surface layer
Figure below shows SEM micro images of an MCA processed surface (120° and cross section):
The main objective of this project was to bring the MCA laser ablation process to maturity and demonstrate commercial integration of this process with BAE Systems’ brazing processes using LMMFC-D tooling and missile radome hardware. A pre-commercial prototype of a IRBAS (Intrinsically Reinforced Barium Aluminum Silicon) ceramic radome brazed to a Ti alloy attachment ring was planned to be demonstrated and tested. The furnaceless brazing was planned to be performed by using the BAE Systems’ Reactive Nano Technologies NanoBond® process [2,3,4].
The figure below shows the progress in formation of the MCA structures on Ti alloy samples by varying the laser process parameters and processing ambient:
Figure below represents the progress of MCA processing on IRBAS ceramic by varying laser processing parameters and processing ambient:
During the first project year the following milestones have been met:
- Further optimization of the MCA fabrication process
- Theoretical and experimental modeling of the MCA geometry in order to improve the wettability by the brazing alloy
- Improvement of the laser system capabilities
- Fabrication of MCA on BiT coupons
- Transfer of the technology to a commercial vendor
- Chemical characterization of the MCA structured Ti alloy
During the second year of the project the following milestones have been met:
- New laser setup and repairs
- Further optimization of the MCA fabrication process on IRBAS
- Fabrication of MCA on the IRBAS BiT coupons using the new Sintec laser
- Tensile stress testing of the 1st batch samples at the University of Houston
- De-bonding of the 2nd and 1st batch samples and refurbishing of the IRBAS coupons
- Bonding of the 3rd batch of samples at BAE Systems
- Tensile stress testing of the 3rd batches of samples at the University of Houston
- Re-bonding of the 2nd batch samples at BAE Systems
- Tensile stress testing of the 2nd batch samples at the University of Houston
- Work with potential commercial vendor on the technology transfer
- Design and assembly of a CNC stage for MCA fabrication on original IRBAS radome surfaces
The figure below shows the fractured samples images and tensile stress testing results for BiT (bond-in-tension) Ti alloy-IRBAS ceramic pre MCA processed samples bonded by using reactive nanofoil technology
The testing results indicate the following:
- The fracture stress numbers are consistent for most samples and indicate a range of maximum loads at fracture from 3018 psi to over 7300 psi (3687 psi average)
- Most of the samples fractured at 100% area through IRBAS ceramic and correlate well with the value of the average strength
- Two samples out of 4 partially fractured through the bond indicate higher than the average strength
- One sample out of 4 partially fractured through the bond indicates strengths within the average strength
- Only one of the 4 samples partially fractured through the bond indicates strength below the average value
Drastic Improvements in Bonding of Highly Dissimilar Materials
The objective of this project is to achieve a revolutionary improvement in the bonding between highly dissimilar materials used for various industrial, aerospace and military applications. The proposed pulsed laser assisted Micro-Column Array (MCA) technology is applicable to both adhesive and brazing types of bonding.
Achievement of a dramatic increase in the bond strength in the metal alloy/adhesive (braze) and composite/adhesive (braze) interfaces of existing advanced materials and structures suitable for advanced industrial applications is the main goal of this project, which will also focus on implementation of the proposed technology for new materials developed up to date and scaling of the proposed technology to large area and complex shape ceramic- and composite-to-metal structural joints .
The goals of this effort was directed towards demonstration of the feasibility of the MCA technology applied for material joints used in specific applications. During the course of the project MCA processing for previously used silicon nitride based ceramic and titanium alloy has been optimized to achieve higher cost effectiveness.
Material selection was performed based on potential applications that have been researched during the project. Coupons of selected materials, such as silicon nitride based ceramic, silicon carbide based ceramic, titanium alloy, Kovar, silicon, and stainless steel have been machined, MCA processed, bonded by using brazing and adhesive bonding. Computer based simulation and bond modeling performed for selected materials indicated advantages for employment of the MCA technology.
The project partially addressed the issues related to brazing of the selected metal alloys and low ductility of the silicon carbide based ceramic affecting the stress testing results. The MCA processing of high thermal conductivity materials, such as silicon and cast aluminum has been initiated. Concepts for employment of computer controlled translation stage in order to process complex shape samples have been developed.
The figure below shows SEM images of: Si3N4 samples processed under Argon environment:
The figure below shows SEM images of: (left) Ti alloy samples processed under ambient air, (right) magnified image of MCA.
The figure below shows SEM images of: (left) Stainless Steel samples processed under ambient air; (right) magnified image of MCA.
The figure below shows SEM images of: (left) SiC samples processed under argon ambient; (right) magnified image of MCA.
On average the MCA processed surfaces had higher bond strength than the non-MCA ones . For example, bonding of Ti metal to SiC ceramic gave almost 100% increase in bond strength from 1000 Psi to over 2000 Psi as shown in the figure below:
In the case of Kovar/SiC brazed structures, the non-MCA (reference) coupon bond strength could not be tested because the SiC ceramic in both cases broke away leaving a very small ledge from which to shift the ceramic. Several attempts to break the bond only resulted in damage to the testing jig, unfortunately we do not have a reference point to compare the bond strength of the Kovar/SiC bonded coupons. The bond strength of the test coupons brazed with MCA structured surfaces on both sides is about 2,596 Psi and the test data is shown in the figure below:
MCA structured SiC and Kovar coupons along with MCA structured SiC and Titanium coupons were bonded all in the same batch. The samples were then stress tested and the results shown in the figure below clearly indicate an increase in the bond strength with the MCA structured surfaces:
Since adhesive practically does not react with the metal and is thus independent of the metal type but only dependent on the surface, Ti/SiC coupons were bonded and BiT tested. Figure below shows the BIT test results for the MCA structured Ti/SiC coupon vs the reference, with over 300% increase in the bond strength:
Overall, it was shown that the MCA structured surfaces have a significant improvement of bonded metals to SiC based ceramic coupons. With improvement of bond strength, being the highest for the bond-in-tension configuration, with over 300% improvement, and the improvement of bond strength in the shear test configuration being greater than 100%.
The efforts described in the above projects resulted in a US patent .
 E.G. Baburaj, D. Starikov, J. Evans, G.A. Shafeev, and A. Bensaoula. “Enhancement of adhesive joint strength by laser surface modification.” International Journal of Adhesion & Adhesives 27, 268–276 (2007). http://www.sciencedirect.com/science/article/pii/S0143749606000686
 D. Starikov, N. Medelci, S. Paranjape, F. Attia, B. Eranezhuth, C. Joseph, and A. Bensaoula. Enhanced Metal-Ceramic Brazed Bond Strength Using Micro/nano structured Surfaces and Nanofoil Technologies. 4th U.S. Air Force-Taiwan Nanoscience Initiative Workshop. Houston, TX. February 8-9 (2007).
 D. Starikov and N. Medelci, S. Paranjape, F. Attia, B. Eranezhuth, C. Joseph, and A. Bensaoula. Employment of Micro-Column Arrays for Bonding of Highly Dissimilar Materials. UH – DoD Research Conference. UH Hilton Hotel & Conference Center, Nov. 1-2 (2007)
 E.G. Baburaj, D. Starikov,, S. Paranjape, and A. Bensaoula. Improvement of Adhesive Bonding between Similar and Dissimilar Materials with Micro-Column Arrays Formed by a Laser Assisted Surface Modification. Society for Advancement and Process Engineering (SAMPE), Baltimore, MD, June 3-7 (2007). https://www.researchgate.net/publication/286587952_Improvement_of_adhesive_bonding_between_similar_and_dissimilar_materials_with_microcolumn_arrays_formed_by_a_laser_assisted_surface_modification
“Improved Nanoreinforced Composite Material Bonds with Potential Sensing Capabilities”; David Starikov, Clyde A. Price, Michael S. Fischer, Abdelhak Bensaoula, Farouk Attia, Thomas A. Glenn, and Mounir Boukadoum., Sensors & Transducers Journal, ISSN 1726-5479, Vol. 13, p117 (2012).
 “Improvements in Bonding of Silicon Carbide Ceramic to Metals”. D. Starikov, R. Pillai, T. Glenn, J. Gandhi, A. Price, R. Delaney, A. Bensaoula. International Journal of Materials Engineering, 4(6): 196-202 (2014). https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0ahUKEwjgjZDg9YXRAhXrAcAKHYoNCLgQFggaMAA&url=http%3A%2F%2Fwww.sapub.org%2Fglobal%2Fshowpaperpdf.aspx%3Fdoi%3D10.5923%2Fj.ijme.20140406.03&usg=AFQjCNFCE1BHmGe8B1KTHOjM3SiQ7j-KKA&bvm=bv.142059868,d.d24&cad=rja
 D. Starikov and A. Bensaoula “Method of bonding solid materials” US Patent 8,962,151 (2015).