Understanding Paper Machine Ceramics: History, Safety, Failures and Causes

The use of ceramics on dewatering elements in the forming and press section of a modern-day paper machine was introduced over 60 years ago. The use of ceramics on paper machines began in the mid-1960s. Ceramics were chosen primarily because they offer superior wear resistance over materials such as polyethylene or Teflon™.  Wear resistance is the main driver of overall paper machine performance because it allows the machine to maintain constant process conditions for a longer period. Key factors in the papermaking process such as dewatering and agitation are reduced as blades wear leading to decreased paper quality and production can suffer.

Construction of Ceramic Blades

Paper machine ceramic blade construction has come a long way since the 1960s when suppliers resorted to many different types and methods for affixing the ceramic segments to a base holder. Today, all suppliers of paper machine ceramic blades have settled on a similar process of bonding the ceramic segments to a pultruded fiberglass resin (FRP) base substrate. The general differences between suppliers are the ceramic shapes used, bonding method and prep, bonding compounds, and the grinding and finishing process. Sourcing of the ceramic segment materials, the bonding compounds, and base substrates is done in an open market so basically every paper machine ceramic blade supplier is buying their materials from the same few sources.

Ceramic Material Choices

There are a variety of ceramic materials suitable for paper machines, however, the time-tested and proven choices include aluminum oxide (high grade), silicon nitride, and silicon carbide.

Zirconia, once on this list, was considered a premium material because it was the toughest of all ceramics and resisted chipping. However, it has since fallen out of grace with most suppliers because it lacks wear resistance and can sharpen to the knife edge (zirconia is often used to make knives). There are also blended materials still being offered such as aluminum oxide blended with zirconia, a.k.a. toughened aluminum oxide. Blending may sound like a clever idea, but there are pros and cons. In this case, toughness and resistance to chipping were improved; however, the wear resistance suffered and was less than that of standard aluminum oxide. Caution should always be taken with any blended materials because the cost of a single issue or failure will outweigh the monetary savings.

Handling Ceramic Blades

The handling of ceramic blades is often misunderstood and is the root cause of many failures. Ceramic manufacturers take great care in transporting ceramic blades throughout the building, finishing, and packaging processes. Good suppliers use transport dollies and roller tables to move newly constructed blades from set-up to finishing. After finishing, dollies are used to move blades to packaging, where they are supported and cushioned into carefully arranged compartments in the shipping crate. Shipping crates with specially reinforced sides to resist bending is a best practice. Steel-reinforced crating is considered the ultimate in protection. Center or single lift points are prohibited, and if lifting options only offer a single pick point then a strongback must be used.

Installation of the ceramic blades onto the paper machine is where compromise most often occurs, which can result in unseen damage exposed months or years later. Blades should always be slid directly out of a crate which is equally supported at 1/3 points across its length. Avoid hand-carrying blades and using ladders or manlifts to raise blades to the installation position. Blades should never be pounded onto tee bars with a mallet or dead weight because it can cause a variety of issues from cracking and chipping to bond failure. Usually, issues with poor blade fit, especially if they slide partway onto a machine without issue, are due to tee bar problems, such as a build-up of debris or straightness and flatness.

Failures and Causes

Crack Propagation

Crack propagation is when a fracture has occurred in a ceramic due to impact, stress, thermal stress, and thermal shock. The fracture could be undetectable for a period; however, through thermal cycling of the paper machine from start-up to shut down, the fracture continues to open until it cracks through and becomes visible or destructive to the paper machine's clothing.

Stress Cracks, Thermal Stress Cracks, and Thermal Shock

Each type of crack associated with paper machine ceramic blades has a different underlying cause.

Stress cracks are typically found in the machine direction and are caused by exceeding the bending limit on the ceramic segment. Mishandling during installation is most often the cause. In general, there is no great concern for full-on failure of the blade with stress cracks. Think of a stress crack like additional joining elements.

Thermal stress cracks are typically in the machine cross-direction. These cracks occur when the end loading on adjoining segments exceeds the compression limit. Once the compression limit is reached, the segment pushes itself apart in the direction that is unrestrained and along the weakest point in the cross direction. Because the segments are adjoined, they are restrained on the ends. The leading and trailing edges are unrestrained and as such the segment fractures along this path. Blades with cross-direction thermal stress cracks should be replaced especially if segment pieces begin to dislodge.

Thermal shock is the destruction of the individual ceramic segments that make up the blade assembly. Thermal shock occurs when there’s a rapid change in temperature on and within the ceramic segment itself. This phenomenon happens through improper application of ceramic materials and improper operation of the paper machine. Frictional heat created by the moving fabric on poorly lubricated ceramics is the primary cause. Aluminum oxide fails in these extreme conditions because it possesses lower thermal shock resistance. Silicon nitride and silicon carbide have a much higher thermal shock resistance value and are better suited to applications where high heat can be generated. Curved surfaces and high vacuum levels require more thermal shock-resistant ceramic. Blades with thermal shock issues should be replaced.

Chips

Chips are cracks that have fully developed to expose a parcel of the ceramic segment and are no longer attached to the main element. There are two main types of chipping: impact and corner chip.

Impact chipping is when something has come in contact with the segment to cause the exposed ends of the ceramic to break off. There are many sources for impact chipping, but most often it is carelessness around the ceramic, such as laying tools, rigging down, or walking haphazardly across the drainage structures without protection on the blades. Impact chips can show up immediately or it may take a bit of time for crack propagation to fully take place. Blades with impact chipping can often be honed in the field without needing immediate replacement.

A corner chip occurs when two segments come together at a joint. The chip is usually seen on one side of adjoining segments, but it could appear on both sides. Often a corner chip is a result of blade handling. Bending or twisting of the blades causes a fracture where two ceramic segments adjoin when the bending or twisting load exceeds the fracture limit of the ceramic segment. Like impact chips, corner chips may not show themselves immediately because the fractures may not be complete until the thermal cycling of the paper machine takes place. Blades with corner chips can often be honed in the field without needing immediate replacement.

Piano Keying

Piano keying is a failure of the chemical bond between the ceramic segment itself and the base material substrate (fiberglass in most cases). Piano keying is a serious issue that if left unchecked can result in ceramic segments dislodging completely from the base. This can cause catastrophic damage to machine clothing, other ceramics, rolls, and other equipment. Bond failure can be brought about in several ways including blade workmanship, improper prep of the bonding surfaces, and/or defective adhesive. Bond can also be compromised by overheating ceramic or if there is excessive force applied during installation. For example, using a mallet or dead weight on the end of the blade to pound it onto a tee bar. Blades displaying widespread piano keying should be replaced.

Spalling

Spalling is a condition where the surface of the ceramic breaks away in layers, leaving pockets. Spalling is often brought about by overheating ceramic. Spalling is most often a sign of thermal shock in nitride and carbide-type ceramic blades. Blades with widespread spalling should be replaced.

Scoring

Scoring is a wear-related condition where distinct lines are visible on the wearing surface of the ceramic. Most often scoring will affect the entire cross-direction length of the blade inside the fabric run. Scoring can be brought about by improper fabric cleaning or a chemical upset in the papermaking process. Scoring happens when debris or fillers adhere to the forming fabric; thus, the fabric becomes a sanding belt. After scoring has occurred to the ceramic, overall fabric life will be decreased because the finish has been compromised. Scoring typically affects blades on the dryer positions of the forming section. Whole groups of blades need to be replaced when scoring occurs.

Erosion or Orange Peel

Erosion or orange peel is a surface condition of ceramic where the fine grain particles of ceramic that make up the segment are gradually worn away leaving micro pockets in their absence. The filler's age and usage are the general cause. Overall, the surface finish will become very poor and clothing life may be compromised. Blades displaying poor surface finish should be replaced.

Deckle Edge Wear

Deckle edge wear is when the area just inside the deckle on a vacuum unit wears at an accelerated rate from the rest of the ceramic of that same blade. If left unchecked the deckle edge can wear down to the base substrate material (fiberglass) and can cause clothing edge wear issues. Deckle edge wear can be caused by an overly dry edge or poor edge cleaning of the forming fabric. Blades with deckle edge wear will need to be replaced even if the rest of the blade within the sheet web is still looking new. Countermeasures for installing lubrication showers and lubrication deckles should be employed to avoid a continued issue with deckle edge wear.

Sharpness and Saw Toothing

Sharpness and saw toothing are both wear-related issues most evident on the leading edge of blades that have a doctoring or skiving angle. Sharpness can occur on the trailing edge of blades but is usually less serious because most blades have a departure angle that is usually 90° or more. Saw toothing is when the sharpness becomes extreme enough that the edge becomes thinner and begins to break away. In the case of either condition, field service can usually hone the affected blades, but if wear is too extreme then replacement should be considered.

Base Substrate Material Deterioration

During blade construction, most suppliers of ceramic dewatering elements use commonly available base substrate materials. This base material is known as fiberglass pultruded resin (FRP). As this base material ages, and goes through repeated thermal cycling, and exposure to the harsh chemicals in papermaking, the integrity of the FRP can become compromised. Other factors such as impact, and improper handling can cause issues to crop up. Typical issues might be cracked tee slots, damaged pull holes or erosion of the material from water/fiber impact. During normal blade inspections, the base of all blades should be looked at, and if significant issues are found, the blades should be replaced.

Debris Build Up

Debris that builds up with discoloration on the surface of ceramic blades can be a sign of other paper machine issues. If you see build-up, there is usually an issue with how the blade is sitting on the tee bar or another mounting device attached to the structure, or the issue could be the structure itself. Check the tee bars for cracking especially if they are made from FRP. Blades that are building up with debris are usually not fully in contact with the forming fabric, and as such would contribute to CD moisture and weight profile issues of the finished paper.

Slot Plugging

Slot plugging is more of a process issue than a ceramic issue, but it signifies a drainage issue that can create future problems with the ceramic. Slot plugging is an indicator of the lack of drainage or vacuum capacity. Without sufficient drainage, ceramics are likely to be improperly lubricated causing heat build-up, which leads to thermal cracks or thermal shock.

Deckle Issues

Deckles on low and high vacuum drainage units are often overlooked. Inspections of these structures should verify that all deckles are present between the ceramic blades and there is no significant damage or wear to the deckles. Deckles used near steam hoods should be made from high-temperature polyethylene to ensure they do not warp or melt. The inbound deckle edge should not be sharp which can cause fabric damage.

Tee Bars and Other Blade Mounts

Checking tee bars and other blade mounting methods is another overlooked area until an issue arises. It is good practice to look over all mounting to make sure nothing is broken or loose. Typically, tee bars are made from stainless steel; however, there are several machines and machine builders still using FRP for tee bars. FRP tee bars are inexpensive to install, but their usage comes with a long-term cost because they will break leading to a costly crash.