- 10 April 2006 -
'Super Finishing' Gets Even Better
By Lior Ben-Tsur, lior.b@Frisco.com

The need for new and more advanced solutions for minimizing the process time required to improve metal surfaces and reduce friction has been accelerated in recent years for two main reasons: fierce competition between engine manufacturers and automotive makers, which constantly requires suppliers to lower their prices, and maintaining long service life and durability in spite of reduced prices.

This article discusses a new approach that addresses both issues in a single process that saves time and cost.

Introduction
Surface finishing and surface treatment have been a challenge to the machining industry and material engineers in light of the burdens related to application costs and the time it takes to super-finish a coated mechanical part.

At the same time, U.S. Environmental Protection Agency (EPA) regulations¹ impose higher standards of engine emission, which force engineers to use coatings on parts that operate under extreme operating conditions in the engine. Thus, the need for coatings becomes an expensive necessity.

These contradictory requirements drive the research and development towards solutions that will enable price reduction throughout the super-finishing process and will fit mass production applications. The key to the development presented here was based on combining two main factors:

• A process that can be used in existing production machines;
• and a solution that will dramatically shorten the super-finishing process time on coated (hardened surface) or uncoated metal surfaces.

Table 1: Surface Roughness (Click to enlarge)

The available methods known to the industry are using several steps/stations to improve surface roughness from N8 (after coating) to N2 (where the Ra level is required in order to lower friction). (See Table I.)

The time it takes to super-finish a surface with a hardness of Rc 68 is substantial, considering the hardness of the metal surface and the abrasives available in the market. The research was aiming to find an alternative to the available super-finishing methods by providing a simple and cost-effective technique for super-finishing.

FriCSo Super-Finishing (FSF)
FriCSo Super-Finishing (FSF) is a standard lapping process where a FriCSo-developed patent-pending consumable polymer is used together with abrasive paste. The FSF reduces friction by enhancing the micro-characteristics of the metal surfaces.

When the polymer tool is used with the abrasive paste in the lapping process, several unique phenomena are achieved: a very flat, smooth surface with dramatically increased sliding distance until seizure; reduced friction and energy losses; and reduced wear between friction pairs. In addition, it takes significantly less time to achieve an N2 level of surface finish (Ra = 0.05) and it is done in a single step. The FSF process creates a sustained, consistent surface².

FriCSo Polymer Lapping Technology
Any lapping process, including the FSF process, involves three elements: the part (or specifically the surface) that should be lapped; the abrasive particles that actually machine the surface and reduce its roughness; and the lapping tool that exerts the force required to affect the machining operation with the abrasive particles.

In a regular lapping process, where the lapping tool is made of metal (usually cast iron), the abrasive particles penetrate the metal tool and are held there firmly. This gives the abrasive particles the ability to abrade the surface until they wear, chip, and stop abrading it. This is the reason why regular lapping requires several steps to smooth out a surface. In each step, ever smaller abrasive particles are used to achieve reduction in surface roughness. 

In the FSF process, the tool is made of a specially designed polymer. This affects the lapping process in two ways.

First, the polymer tool (that is weaker than the metal tool in regular lapping) does not hold the abrasive particles as firmly as the metal tool. Once the abrasive particle loses its cutting point (due to wear or chipping), there is an increase in friction force between it and the surface, which causes the abrasive particles to tear away minute particles from the polymer and swivel. This allows a new cutting point to touch the surface and continue the abrading process.

Figure 1: Regular super-finishing.

Figure 2: FriCSo super-finishing, single, process.

Figure 3: Bearing area curve (BAC).

Figure 4: Test results (sliding distance until seizure; hard steel on hard steel.)

This abrasive particle rotation repeats continuously until the particle loses all cutting points and becomes spheroid.

Once this stage is reached, the hard spheroid begins to roll on the steel surface and compact the surface by plastic deformation. This mechanism of surface smoothing using both machining and plastic deformation is unique to the polymer lapping tool.

Second, the small minute particles torn out during the abrasive particles' rotation are fragments of the polymer lattice, and they react with the steel surface.

Effect of Polymer Lapping on Surface
Pictures were taken with an atomic force microscope (AFM). Figure 1 is the result of a commercially available super-finishing process with results of: Ra = 0.07 µm and Ry = 0.25 µm. Figure 2 is the result of FSF with results of Ra = 0.02 µm and Ry = 0.13 µm.

FSF forms new surface characteristics. The roughness average (Ra) is improved from Ra 0.4 to Ra 0.02 microns in a single step. The surface hardness is increasing beyond 1,100 Hv and the bearing area curve (BAC) characteristics are significantly improved, while surface geometry remains unchanged (see Figure 3).

The BAC in the upper strip shows a surface after regular super-finish has much higher peaks, compared with the lower strip report, which shows a surface after FSF. FSF can be easily adapted to lapping machines and may be applied to cylindrical parts, flat surfaces, and on a variety of metals. It is viable for mass production or small quantities, addressing the needs of a variety of industries, such as automotive, heavy-duty, agriculture, and hydraulics. The FSF is environmentally friendly and does not use hazardous materials or produce waste products.

Tests have shown that parts treated with FriCSo technology benefit from dramatic improvement in service life compared to parts treated with grinding and regular lapping³.

In one of the tests, several steel samples were lapped, either by using a regular lapping method with a cast iron tool, or the FriCSo polymer lapping tool. All samples were machined with the same abrasive paste. After lapping, the samples were analyzed by X-ray photoelectron xpectroscopy (XPS)⁴.

Conclusion
The polymer-based surface finishing technique (FSF) meets new market demands both in superior surface results and in competitive cost-performance, compared with existing processes. In addition, the FSF reduces friction, wear, and energy consumption, and leads to savings in service and maintenance costs.

References

1. Environmental Protection Agency—emission standards for heavy-duty highway engines, model year 2007 and later.
2. Results of AFM (atomic force microscope) test indicate the improvement in topography of the surface created by FriCSo polymer lapping, The Material Engineering Department, Technion, Israel Institute of Technology, Haifa, Israel; April 2005.
3. Results of polymer lapping performance test examining connection between COF and sliding distance, Falex Tribology Testing Laboratory; August 2005.
4. Results of XPS test indicate that there is a chemical bonding between the FriCSo polymer and the metal’s nanometric layer, The Solid State Institute, Technion, Israel Institute of Technology, Haifa, Israel; July 2005.

Lio Ben-Tsur can be reached at lior.b@Fricso.com.