I-Rotary Engine Coatings
The I-Rotary Coating System has been designed and manufactured to achieve a high quality interface surface between the sealing grid and the rotary engine internal envelope. The guidelines for this I-Rotary coating system are:
Achieve a very low wear rate
The coating has to perform for many hours of use in a high level endurance racing environment.
Exhibit an extremely low Coefficient of Friction (COF)
It is very important that these coatings provide a low COF to extract the most power and achieve the best fuel consumption.
Demonstrate a high level of reliability
Hard surface wear resistant coatings are well known to be susceptible to various forms of failure such as by debonding from the surface or by fracture. A requirement of the I-Rotary Coating System is to minimize or eliminate the chance of failure due to delamination. This failure mode can be caused by material compatibility issues such as a thermal expansion mismatch between the coating and its substrate or due to poor craftsmanship practices during the application of the coating.
Another key requirement of the I-Rotary Coating System is to be reliable even when this interface is exposed to a severe impact event, for instance, when hit by an out of balance rotor caused by an engine spun beyond its maximum revolution limit. This condition unfortunately can happen in a racing car environment, when a lower gear is incorrectly selected either by the driver or due to a gear box malfunction.
For instance, the original wear resistant coatings provided by Mazda for the side housings of the 13J 4-rotor and the R26B 4-rotor prototype race engines had excellent wear resistance for very long endurance racing applications. These surfaces, however, were prone to cracking and/or shattering into several small hard ceramic-like pieces during extreme engine events. These small hard pieces would cause the destruction of the sealing grid. If this debris contaminated the oiling system it would destroy most of the engine as well.
The development program of the I-Rotary Coating System for the side housings of the rotary engine is described below.
1987 - A Challenging Coating Repair Experiment
In 1987 we were selected by Rick Engman of Downing/Atlanta, Inc. to assist in the repair of an unavailable and rare factory nitrided cast iron side housing for a Mazda 10A rotary engine. This side housing had suffered a large gouge measuring 1.25 in. x 0.5 in. wide ~0.125 in. deep (~30 mm long x 13 mm wide by ~3 mm deep).
The repair procedure chosen for this project had to fill in this gouged cavity with a reliable coating which would be very stable and would not separate or delaminate from the repaired area during use for the life of the engine. It also had to be compatible with the sealing grid, and wear at the same rate as the nitrided surface.
An additional request made by Mr. Engman was that the coating had to be lapped or ground using standard tooling rather than the diamond tooling typically required for very hard coatings. This request was based on the client’s desire to finish the repaired housing in-house.
Several materials and processes were evaluated for this challenging project. As far as we are aware, there has never been an issue with this engine, which is still in use today.
1993-1997 - The IMSA WSC Prototype Race Car Experiment
In 1993, we were given the opportunity by Downing/Atlanta, Inc. to develop a coating system which could be applied to the internal wear surfaces of used nitrided cast iron side housings from a Mazda 13G competition 3-rotor engine. These engines were to compete for several seasons in the International Motorsports Association of America (IMSA), World Sport Car series (WSC) prototype class.
The development of this I-Rotary Coating System involved the study of over 100 variations to achieve the desired specifications of low wear, low friction coefficient and high reliability. More than 60 side housing surfaces were coated for the Mazda 13G 3-rotor engine during this effort. The parameters studied for this project included:
- The coating alloy chemistry
- The size, geometry and consistency of the coating powders
- The process of how these powders were mixed
- The humidity of the powders during the spraying process
- The method used to spray the powders
- The type of gases used
- Mechanical wear, bond and friction coefficient tests
The initial sets of side housings performed as designed in the racing engine environment. However, it was found later that one of the coated surfaces of a spare set of side housings began to blister after a year in storage.
It was determined that this blistering of the coating was the result of moisture reaching the cast iron through pores in the coating. This mechanism resulted in these areas underneath the coating to rust which caused it to blister. This is similar to what happens to the painted surface of a steel car body when the metal underneath the paint rusts.
The spare set of side housings had been stored in a room with no environmental control which at some point may have exposed these housings to high humidity.
While the blistered coating rendered the side housing unusable, it should be noted that the coating never cracked. The fact that it did not crack was an indication of the toughness of the I-Rotary coating.
All of the coatings which had been used up to that point were designed to exhibit a density of at least 98+%. Any coating with this level of density is considered to be capable of providing high structural integrity.
The coatings were also designed to exhibit various levels of porosity as an oil retention mechanism to lower the friction as well as minimize the wear of the coating and the sealing grid. However, any coated product should also be able to withstand various ambient conditions of a storage facility, such as high humidity, with no issues.
A study was started to evaluate what effect the exposure a long term environment of high humidity would have on the selected I-Rotary coating. Several test pieces of plain steel, 4 in. x 4 in. x ~1/2 in., were coated with the originally chosen coating compound. The only variables in this experiment were the characteristics of the chosen raw materials, the parameters of the process to apply the coating and the density level.
The final selection of the coating was simple since only one of the coated surfaces survived this experiment with no rust, which is a clear indication that no moisture had reached the steel backing plate. A few results of this experiment are shown in image below. A section of the selected test sample was cut for further microstructural analysis.
A Few Coating Test Samples from the Corrosion Test
The I-Rotary coated side housings were used during the 1994 IMSA season by the Downing/Atlanta, Inc. race team in their quest for the IMSA WSC Championship which was won by Jim Downing and Wayne Taylor. Additionally, these side housings continued to be part of all of the race engines used in subsequent seasons which included special races such as the 24 hours of Le Mans, the 24 hours of Daytona and the 12 hours of Sebring.
The reliability of these I-Rotary side housings was excellent. There were no issues with any of these coated surfaces due to high wear or delamination caused by poor craftsmanship or material imperfections during this period of professional competition. There were also no fracture failures as a result of rotor impact, even though there were many of these undesired conditions in over 3 seasons of highly competitive world endurance racing. Additionally, this coating provided a measurable performance advantage due to their low friction.
As far as we are aware, there has never been a reported side housing failure caused by coating malfunction in any of these engines. The engines with the I-Rotary Coatings are still being used today in race cars running in historic races.