Micro-friction surfacing on ceramics
TWI Bulletin, March - April 2010
Using the mechanical and thermal properties of ceramics often demands joining them to other materials, but how?
Abbas Mirlashari joined TWI's Ceramics section in 2007 after completing a BEng (Hons) degree in electrical and electronics engineering. He has worked on a number of projects investigating ceramic/metal joining techniques such as soldering, brazing and diffusion bonding. He has recently also run collaborative projects investigating transparent conductive coatings such as indium tin oxide.
Ceramic materials are used in a wide range of applications, from cutting tools and abrasives for polishing to space shuttle insulation tiles and electronic components. The diverse range of applications demonstrates the differing material properties exhibited by ceramics. To use the desirable properties of ceramics, such as excellent mechanical and thermal properties, into components they often need to be joined to other materials, usually metals. However as Abbas Mirlashari explains, their brittleness, low coefficient of thermal expansion, and high melting temperatures compared with metals has made identifying the most suitable joining method difficult.
Joining methods such as adhesive bonding and mechanical fastening can be used to join ceramics to metals. However, to produce a durable and hermetic bond that can withstand a variety of environments and temperatures a direct bonding technique may be more appropriate. To create a direct bond the ceramic parts are often metallised to provide a transition structure from ceramic covalent/ionic bonding to the metallic bond. This will assure wetting and chemical adherence between ceramic and metallic parts.
There are various techniques available to metallise ceramics, such as, Molybdenum - Manganese process, active metal brazing, gas metal eutectic (direct bonded copper), chemical vapour deposition and physical vapour deposition. However, these metallisation techniques can be complicated, time consuming and expensive. Hence alternative metallisation techniques need to be identified.
Friction surfacing is a solid-state joining method for depositing a coating onto a flat surface using a consumable. This has been used in the past for depositing metal coating onto metal substrates for applications such as corrosion resistant coatings. However, friction surfacing has not been used to metallise ceramics. The process works by forcing a consumable to move parallel to the surface of the plate whilst simultaneously rotating and under an axial load, as shown in Figure 1. Therefore, the aim of this work was to establish the feasibility of metallising ceramics using the friction surfacing process.
Fig.1. Principles of friction surfacing
The aim of this work was to expand the scope of friction surfacing by depositing alternative metals such as copper onto ceramic substrates using the friction surfacing process.
Objectives
- Demonstrate the feasibility of joining aluminium to alumina using the friction surfacing process
- Demonstrate the feasibility of joining copper to alumina using the friction surfacing process
- Demonstrate the feasibility of joining aluminium to float glass using the friction surfacing process
Materials
The following consumables and substrate were used:
- Aluminium rod (99.5 wt% Al), 3 mm diameter
- Copper rod (99.9 wt% Cu), 3 mm diameter
- Alumina substrate (96 wt% Al2O3, and 4 wt% MgO, SiO2 and CaO), 50 x50 x0.635 mm
Experimental approach
A micro-friction stir welding machine was used to perform the friction surfacing trials. The machine has a robotic arm which can move in the X and Z axes and also has a base plate which travels in the Y axis. The traverse speed is controlled from the machine software. A basic weighted jig was designed to fit on to the base plate and introduce load control to the system. The jig consisted of a wire run over a pulley, connected to the plate which holds the samples on one side, and is loaded with weights on the other. The weights can be varied to control the applied force. The jig can be seen in the foreground in Figure 2. A recess was machined at the centre of the base plate to hold the substrate in place.
Fig.2. The friction stir welding machine with the friction surfacing jig in the foreground
The key parameters that could be varied during friction surfacing to establish the feasibility of depositing aluminium to aluminium oxide were traverse speed, rotation speed and load. The widest available range of rotation speed and traverse speed settings were used whilst the load remained fixed at 20N. The rotation speed settings used were 8000, 13500 and 24000 rpm, and the traverse speed settings used were 0.5, 0.05 and 0.005 m min-1. At each setting a 20mm long track was deposited.
Results
Joining aluminium to alumina
The results, as shown in Table 1, show that it is feasible to friction surface aluminium onto alumina at the selected rotation and traverse speed settings. It was found that increasing the rotation speed does not have a significant effect on the amount of material deposited and the width of the track. However, decreasing the traverse speed resulted in wider tracks being formed, and more material being deposited.
Table 1 shows the tracks that were deposited at different rotation and traverse speed settings
Traverse speed
m min-1 | Rotation speed rpm
8000 |
13500 |
2400 |
| 0.5 | | | |
| 0.05 | | | |
| 0.005 | | | |
A friction surfaced sample was sectioned, mounted and polished and the bond line was analysed using scanning electron microscopy, as shown in Figure 3.
Fig.3. An electron micrograph of a section through a friction surfacing bond showing the interface between aluminium and alumina
Joining copper to alumina
It was not possible to friction surface the as-received copper rod onto the alumina substrate using the range of parameters available. This was thought to be due to the hardness of the copper rod. Therefore, the copper rod was annealed (heated to 700°C at 10°C/min and held at 700°C for 20 minutes) and then quenched (in water) to reduce its hardness.
The light microscopy images (Fig.4a and 4b) show the grain structure of the copper rod before and after the heat-treatment. The Vickers hardness value of the rod was measured (Vickers load 5 kg) before and after the heat-treatment. The as-received copper rod had a hardness value of 123 HV and after the heat-treatment the hardness value was 62 HV.
Fig.4. Light microscopy images of the copper rod a) before; and
b) after the heat-treatment
The heat-treated copper rod was used to try and deposit a track using traverse speed of 0.1 m min-1, rotation speed of 24000 rpm and load of 10 N. It was possible to deposit a copper track onto an alumina substrate using friction surfacing, as shown in Figure 5. This suggests that the hardness of the consumable is an important factor when trying to deposit a layer.
Fig.5. Image of a copper track friction surfaced onto alumina, after heat-treatment of the copper
Joining aluminium to float glass
It may be of interest to companies producing sealed vacuum glazing glass panels to metallise the glass edges using friction surfacing. Currently, organic materials are used to seal the panels but they leak slowly. Friction surfacing allows for a subsequent joining method that will resolve the leakage problem.
Following the initial trials the feasibility of joining aluminium to alumina was established as shown in Figure 6. However, cracks were detected on the glass substrate when the track was analysed through the glass substrate using light microscopy, as shown in Figure 7.
Fig.6. Shows three aluminium tracks deposited onto float glass using the friction surfacing process
Fig.7. Shows that cracks were present on the glass substrate following the friction surfacing process
The cracks occur at the start of the tracks, when the glass substrate and the aluminium consumable make contact. The initial point loading on the substrate may result in the crack formation. Therefore, it may be feasible through variation of the parameters or alternative jig set-up to overcome the initial shock and prevent the cracking of the glass.
Conclusion
The friction surfacing process was used to metallise alumina substrates using aluminium and copper consumables.
It was feasible to deposit aluminium tracks onto alumina substrates using the friction surfacing process. Increasing rotation speed did not have a significant effect on the amount of materials deposited and the width of the tracks. However, decreasing traverse speed results in wider tracks being formed and more material being deposited.
It was feasible to deposit a copper track onto an alumina substrate using friction surfacing. However, the as-received copper rod had to be heat-treated prior to friction surfacing to reduce its hardness.
It was feasible to deposit an aluminium track onto float glass substrate using friction surfacing. However, there were cracks present on the glass substrate following the friction surfacing process.
