Self-Lubricating Surfaces Overview

Hi guys, welcome to our presentation on self lubricating surfaces. This is for Engineering 422 - Surface Engineering’s topic analysis, we’re team eight within the class, my name is avi and I’ll be starting us off for today. We’ll be moving at a rather brisk pace so if you want to take a closer look at anything please feel free to pause things. So let’s get right into things by introducing tribological surfaces, so tribological surfaces are classified as surfaces that react to physical interactions in their environment and change due to it. In recent years we’ve seen a dramatic explosion of research and development of these kinds of materials, you may have seen or heard of self-healing materials, or perhaps even of self lubricating materials.

Our focus 00:41 - today of course is on the latter. Self lubricating surfaces are actually a subset of tribological surfaces, that react to pressure, friction, or other environmental stimulus by excreting lubricant to coat the surface of the material. We can split up self lubricated surfaces into three primary kinds, or classifications: Embedded lubricant materials, materials that utilize lubricant from their environment, and finally materials that use a surface coating of some sort to achieve this property. In addition to this we can also observe self lubricating properties within biology, an example of this is how ducks excrete oil to coat their feathers in order to achieve better buoyancy in the water. So what do these surfaces actually look like? One of the most common implementations is materials that have nano capsules of lubricant embedded within them.

This allows for lubricant to 01:30 - seep through the porous cavities of the material onto the surface when pressure is applied. You can see in our figure here when force is applied to the surface of the material, it excretes a lubricant from within it. Now that we have a basic idea of what self lubricated surfaces actually look like and how they work we can move on to characterizing and quantifying their performance. within this slide we’re gonna be focusing primarily on two things, firstly we’re looking at the performance characteristics of a titanium substrate and then we’re gonna look at the same substrate with a PTFE coating applied to the surface using a plasma electrolytic oxidization treatment we can observe within figure two and three how the frictional coefficient and the average roughness of these surfaces change, potentially contrary to your expectations the average roughness of the surface with the coating applied to it is actually higher. this is likely due to the deposition of the additional material onto the surface.

02:28 - finally we can take a look at the decrease in the wear rate of the surface alongside the change in the frictional coefficient we can see a correlated response in the wear rates of the surface. we can see that the wear rates in the surface with the coating applied to it are decreased almost a hundredfold this has massive implications and the usability and application of these materials but we will leave that for later for now I’m gonna pass things off to Sonia to explain more. Hi everyone my name is Sonia and I will continue our presentation with introducing apparent contact angle of water droplet on self-lubricating surface with oil okay what is important here is critical contact angle of water on oil and gas on oil because Young’s contact angle of them should be less than critical ones and critical contact angle depends on the roughness and solid fraction that we can change with design as you can see in the figure on parts C and E with vanishing contact angle of gas on oil and water on oil we are separating the solid phase from water and gas, if we are not as you can see on figure B and D water and oil can pin on the solid part and increase contact angle hysteresis. Here cassie-baxter state is governing so we can calculate cassie-baxter contact angle for water on oil and gas and oil and use that one for apparent contact angle calculation as you can see in the equation apparent contact angle is not depend just on Cassie baxter contact angles it depends on interfacial tension of three phase for example if we have really low interfacial tension of water and oil we can say apparent contact angle is close to cassie-baxter contact angle of gas on oil another parameter which works to discuss here is a spreading parameter, contact angle is not providing enough information on wetting but how we can figure out, if our lubricant is covering all of the substrate and make a thin layer of film? okay it depends on spreading parameter if we have positive spreading parameter, we have that thin layer of the film. not individual droplets of lubricant on the surface.

here you can see the changing surface 04:49 - energy in this example from steel to fluorinated DLS in using Pio oil on it a spreading parameter is decreasing from positive amount to negative amount and content angle is increasing as well as long as the speeding parameter is positive we are fine now we can talk about advantages disadvantages for actual surfaces for advantages we can say self lubricating surface have longevity due to reduced wear rate and they can repel all kind of the liquids not just water despite usual super hydrophobic surfaces they are optically transparent and they can repulse solid particles and they have anti fouling properties like ice-phobicity and for disadvantages we can say that mostly they are using fluorinated lubricants which produce toxic chemical as by-product they should use initial lubricant which is oil and grease and they have low thermal stability and they are not stable in high shear force and they don’t have high strength because of the way that they are manufactured so they cannot last in high stress and in some cases we will face cloaking problems and etc. how to manufacture self lubricating surfaces and slips they have similar manufacturing but they’re different in first step we have three steps the on the first steps for slips we roughen the flat substrate because the roughness is the key to maintain lubricant on it and for self lubricating surfaces we are making porous substrate in the second step we functionalize the surface to make it active to bond it lubricant and in the last step we infuse the surface with lubricating liquid you can see an example of fabrication of a slips aluminium surface in the figure. What are the challenges that we will face? during using these surfaces and manufacturing them the first one is design how to design and make the droplet to exist in which state (we desire) Cassie-Baxter, Hemi-Wicking, or Wenzel. In order to optimize frictional coefficient and reducing the drag and the other challenge that we are facing here is lubricant how long the lubricant can’t last and what is the effective life-span of the surface because some of the lubricants are degradable. now we can talk about three fascinating examples of self lubricating surfaces, first one is inspired by a living antifouling surface, we can talk about pitcher it has a waxy part inside that is (slippery) and it’s using that for trapping insects inside.

07:51 - Inspire that researchers created 3d vascular system of PDMS and fill all of the porosities with silicone oil infused PDMS here shows really low biofilm coverage compared to PDMS and glass samples Hi I’m Danielle, and I’m going to be talking to you about self lubricated hip replacements the tribological performance of a hip replacement has a high impact on the success and longevity of the replacement. Clinical outcomes of wear testing have found that friction and wear are influenced not just by applied forces and external loads, but also by the material of the replacements ability to build a lubrication film layer. In one of the journals we researched, Chowdhury et al. stated on hip replacement recipients around 50 percent are aged between 45 and 64 who may live for another 25 to 30 years unfortunately the orthopedic implants generally only last 15 to 20 years it is apparent that longevity and hip replacements is an issue which requires innovation towards improvement, surface engineering advances aim to solve this problem through designing hip replacement alternatives which build and maintain a lubrication film layer in the joint. The first self lubricating hip replacement patent we looked at works as a traditional lubricated surface, not a lubricant infused surface.

In this design 09:17 - lubricating fluid fills the textures pillars while also creating an excess lubrication layer on top of the surfaces pillars, this is the hemi- wicking state where the lubricant is in contact with itself. this patent intended to achieve the state through fabrication of metal and ceramic implantable articulation members, with small channels throughout the design, allowing synovial fluid the hip capsules natural lubricant, to move through the articulation member and create the desired excess lubrication layer necessary to mimic natural hip joint mechanics. The second design discussed is also a lubricated surface, not a lubricant infused surface this design utilizes surface engineering by manipulating roughness in the form of micro-dimpling on the surface of the prosthesis to better maintain the lubrication layer. Micro dimples act as reservoirs for the lubricant in the prosthesis which maintain an increased lubrication. It should be noted that a lubricant must be injected into the prosthetic joint between the wear surfaces for this technology to be utilized.

A dimpled hip replacement was found to 10:24 - have a lubrication film layer roughly 3.5 times thicker than the layer on a standard prosthetic a micro dimpled surface on a prosthetic hip replacement allows for less wear and greater longevity of implant through ability to sustain the thick lubrication film layer in the Hemi wicking state on the surfaces of the prosthetic implant. Hi I’m Tyler, and I’ll be talking about self lubricating surfaces in industrial applications. A self lubricating bearing can be better in situations where regular maintenance is difficult or where constantly feeding oil to the bearing is infeasible. There are three main types of self lubricating bearings homogeneous metal composites, metal backed laminates, and homogeneous nonmetallic composites.

Homogeneous metal composites are technically an oil infused surface 11:13 - that slowly emit oil as the bearing heats up metal backed laminates emit solid lubricant such as Teflon or graphite as the bearings heat up, or through friction. And homogeneous nonmetallic composites, which typically bond Teflon to steel using resins, or wound plastics. A lifetime analysis of a self lubricating tungsten carbide bearing versus a standard bearing are shown up in the graph. It shows that for low oscillatory loads the self lubricating bearing had a far greater lifetime however the time to wear out between the two bearings converges as a larger load is applied. Self lubricating bearings are very useful in load load environments where either regular replacement, or regular lubrication is unfeasible; Such as aircraft or remote work areas.

Next we will tie our topic back to concepts that we learn from our ENGR 422 12:14 - surface engineering course. In the literature we found a real-life example of a fabricated self lubricating surface, the surface consists of 1 micrometer square pillars which are 50 micrometers wide and equally spaced. Using this known geometry we found the critical angle, and from that we can see that the critical contact angle is incredibly high. And from that we can say that the surface energetically favors the Wenzel state meaning it will be fully wetted, another example that ties back to our surface engineering course is finding spreading parameters for a common medical material (used in implants like the hip transplant presented earlier.) a cobalt chromium molybdenum alloy is commonly used in the medical field for implants because of its wear and corrosion resistance.

13:04 - We know the quantities of each material approximately (64) percent cobalt, (30)% chromium and 6% molybdenum, as well as the surface energy for each. From this we found the surface energy for the alloy, and after that we selected an example lubricant. We then calculated the spreading parameter for the lubricant on our alloy surface, a spreading parameter greater than 0 means lubricant will cover our surface. Now let us conclude, we introduced the working principles behind self lubricating surfaces, then we laid out the design challenges and implications of the technology. We found three real-world examples of self lubricating surfaces, within nature, in the medical field, and in industrial settings respectively.

13:53 - And finally we directly tied self lubricating surfaces to concepts learned from our surface engineering course. While self lubricating surfaces are already in use in a small scale for specialized applications, as the price and ease of manufacturing declines the technology will be more widely adopted throughout the market. Ultimately the wider use of this technology would make bone and joint replacements longer-lasting and reduced the need for invasive surgery on our elderly population, and would cause considerably less industrial waste generation every year by having bearings wear out far slower. I hope like me, you too are looking forward to what the future holds for this technology. I hope you enjoyed our presentation and I’d like to leave you with a few questions to reflect on. Thank you and have a nice day. .